Method for subsurface aerated treatment of wastewater

[0001] This application is a continuation of patent application Ser. No. 10/292,185, filed Nov. 12, 2002, now abandoned which was a continuation in part of patent application Ser. No. 09/526,381, filed Mar. 16, 2000, now U.S. Pat. No. 6,485,647; and claims benefit of Provisional Application Ser. No. 60/125,070, filed Mar. 17, 1999.

[0002] The present invention relates to the subsurface disposal of wastewater (sewage); in particular, to disposal of wastewater by means of cesspools, septic tank type systems and associated leach fields.

[0003] Subsurface wastewater disposal systems, commonly called septic tank systems or septic systems, are widely used for on-site processing of wastewater from dwellings and other smaller volume wastewater sources. Typically, wastewater is delivered via a wastewater line to a septic tank for primary processing. The septic tank effluent, or wastewater, is flowed to a leaching system for secondary processing by means of distribution pipes. The leaching system, also commonly called a disposal field, leach field, or infiltration field, typically comprises permeable soil of the earth and some sort of excavation in the soil which is filled with stone particulate such as crushed stone or coarse gravel (typically 2.5 cm in dimension) and or a mechanical component, the function of which is to convey wastewater through a conduit, to infiltrate it into the soil.

[0004] The principal function of the septic tank is to effect primary wastewater processing by engendering physical separation and retention of solids which are lighter and heavier than water, typically by settling and baffling. Solids, which settle out as sludge, are mostly decomposed by action of bacteria in a typically anaerobic environment. Gases, which are generated in the process, are vented to atmosphere. The wastewater from the septic tank is typically conveyed to the leach field by passing it through a distribution box and piping which channels wastewater to the leach field trenches, in a predetermined fashion. The wastewater is supposed to be free of solids of significant size. It will contain suspended solids of fine size, microorganisms such as bacterium and viruses, and various chemical constituents.

[0005] The purpose of the leach field is generally to cause the wastewater to be treated or renovated, so it can be benignly returned to the hydrologic cycle that characterizes the movement of water into, through, and from soil beneath the surface of the earth. What follows is a simplified version of certain conventional ways of looking at leach field operation phenomena, to provide a conceptual framework for appreciating the invention. It is not intended to be comprehensive nor limiting.

[0006] As the wastewater travels from within a leach trench and through the soil in a properly functioning system, it is subjected to natural chemical and biological processes within a “zone of influence”, which may extend 30-120 cm from the trench interface with the soil. A traditional leach field is comprised of a trench filled with small (2-3 cm) stone pieces. A perforated pipe runs through the stone, delivering the wastewater along the trench. A popular modern type of leach field comprises a series of interconnected arch shaped molded plastic chambers having perforated walls, such as leaching chambers sold under the Infiltrator brand name. See U.S. Pat. No. 5,401,116 of J. Nichols, and U.S. Pat. No. 5,511,903 of J. Nichols et al. Typically, Infiltrator® chambers are directly buried in a trench in substitution of the stone-and-pipe leaching device.

[0007] The leach field must have sufficient capacity to receive and properly process the anticipated flow of wastewater. The steady state capacity, or the infiltration rate, of a leach field is a function of the resistance to wastewater flow of the surfaces of the trench and the surrounding soil, as such may be influenced by hydraulic phenomena other than permeability, such as capillary action. For illustration here, only the sidewall of the trench will be now discussed. If distilled water is processed in sterile soil of a leach field, the infiltration rate is purely a function of the mechanics and hydraulics of the soil. However, in that wastewater contains organic substances, over time, an active, stable, moist biological crust layer frequently grows on surfaces. Of particular interest is the crust layer which occurs on a trench sidewall and within the nearby soil, especially when the layer tends to block openings in leaching system conduits.

[0008] The crust, also commonly called a biomat or biocrust, is an organic layer, typically 0.5-3 cm thick. It is normally less permeable than the surrounding soil. Thus, the biomat often significantly determines the long-term steady state infiltration capacity of a leach field. The biomat also serves as a filter for bacteria and some suspended solids. In a properly functioning system, the surrounding soil to remain desirably unsaturated and aerobic, thus enabling antibiotic attack of any pathogenic bacteria, and more importantly, chemical reactions involving free oxygen. Biomat is thought to aid in filtering things which enter the influence zone. Nitrogen, discharged in human waste, is characteristically passed through any biomat, predominantly as ammonium (NH₄⁺), to be nitrified, or converted to nitrate (N0₃) form, in the aerobic environment of the influence zone and adjacent soil. Foreign constituents in the wastewater may also sorb and or react with soil constituents; or they may ultimately be only diluted upon return to the ground water. As the wastewater is renovated in the influence zone, it moves mostly outwardly and downwardly toward the ambient water table in the earth. Some water may move upwardly into the vadose above the trench, by capillarity, evaporative-uptake and plant-uptake. It is usually required that the bottom of the leach field trench be a particular distance above the ambient water table, because sub-optimal wastewater treatment conditions exist in the extremely moist soil, the capillary fringe, just above the water table.

[0009] In a properly designed, used and maintained septic tank disposal system, once biochemical equilibrium is reached, the capacity of the leach field remains stable insofar as infiltration or leaching capacity. A long term infiltration rate, or liquid acceptance rate, characteristic soils of southern New England, USA is about 8-32 liters/m²/day. However, too frequently, a septic tank system will demonstrate insufficient infiltration capacity. Typically, a failure is manifested by escape of wastewater to the surface of the soil, or by a substantial backing up of wastewater in the wastewater line. One cause of failure can be gross flow of solids from the septic tank into the leach field piping or chamber system, and blockage of the perforations in such components. The typical best remedy for such is to replace or extend the leach field. Failure can also be manifested by an inability of a given system to handle normal peak loads of wastewater which were handled in the past, and by inadequate purification of the wastewater in the influence zone, resulting in pollution of the groundwater. And, even if a system has not failed, it is desirable to guard against failure by having the greatest economically feasible margin of safety against failure.

[0010] Among the known causes of some failures are the following. The design of the system has become inadequate for the current conditions, either due to growth of a very heavy biomat, a changed character of wastewater, or changed conditions within the soil in the influence zone. For instance, the biological oxygen demand (BOD) of the wastewater may have been increased, or the ambient soil conditions changed, so that the desired biochemical conditions for stable aerobic function in the influence zone are no longer obtained. An accumulation of unreacted wastewater within the influence zone limits oxygen transport. Thus, a cascading type of failure mode may ensue, wherein the influence zone gets bigger and bigger as it gets less and less effective.

[0011] Cesspools, favored in some regions, avoid the use of septic tanks. Untreated wastewater from a source is dumped into and partially treated by natural processes in the pool of an underground pit; and, the the wastewater infiltrates into the influence zone of soil surrounding the pit for further treatment. Phenomena and problems similar to those described for leach fields will exist in cesspool influence zones.

[0012] Thus, there is a need for alternatives to the costly or sometimes physically impossible remedy of adding to or replacing the leaching system. And, if good technology is at hand, the possibility arises for putting in a smaller system initially and reducing cost, for providing greater margin of safety in any given system, or for allowing growth in use of an existing system.

[0013] Various approaches to enhance the capacity of leaching systems have been tried, reflecting different concepts of both failure and remedy. Chemical remedies in the forms of solvents, enzymes, and other proprietary formulations, for deposit into the wastewater line with wastewater, are commercially sold, but most are disdained or ignored by professionals. U.S. Pat. No. 5,588,777 of Laak discloses the injection of soap into the leach field. U.S. Pat. No. 5,597,264 of Laak discloses a method of periodically back flushing the leach field with water. U.S. Pat. No. 4,333,831 of Petzinger describes the type of problem mentioned above, solving it by using evaporation chambers in substitution of any leach field. U.S. Pat. No. 3,907,679 of Yost describes a system in which low pressure air is forced through a septic tank and then into a long coil of wastewater piping, so wastewater evaporates into the air and is discharged to atmosphere. U.S. Pat. No. 3,698,194 of Flynn describes how air is blown into a conduit of a leach field and vented from risers at the remote end of conduit, to cause evaporation of liquid in, and to dry out grease in, the conduit, during periods when the conduit is not being used for wastewater treatment. U.S. Pat. No. 4,013,559 of Johnson describes how air is introduced into the septic tank, flowed through unique vertical concrete panel leaching system units, and then discharged to atmosphere, to encourage aerobic conditions in wastewater within the panels. However, none of these prior art technologies seem to have found wide spread use. Thus, there is a continuing need for new ways to enhance the design and performance of leaching fields, both as they are originally installed and for when there are in need of rejuvenating.

[0014] An object of the invention is to provide means for improving the function of septic tank type disposal systems and leach fields, to remedy failures, or forestall failure, or improve performance, in ways that are economical and practical. A further object of the invention is to effect desirable biochemical and physical conditions within the influence zone of a leach field. A still further object is to provide a way of sustaining or rejuvenating leach field performance while at the same time enabling continuous use of a septic tank type wastewater system.

[0015] In accord with the invention, when wastewater is flowed from a primary wastewater processing unit, such as a septic tank, through a conduit, and into an influence zone in the soil, a significant pressure differential with atmospheric pressure is created, as gas, comprised of air or other biochemically active gas, flows between the conduit and the influence zone, in an amount effective for physical and or chemical change within the zone. In further accord, the flow of active gas is sufficient in amount to make the composition of gas within the influence zone effectively different from the composition which exists therewithin, in the absence of such flowing. Thus, if the leach field is functioning properly, the invention maintains or improves such; and, if the field is failing, the invention will restore part or all of the function. In one embodiment, air flows from a conduit, into and through the influence zone, in the same direction as the wastewater flows. In another embodiment, air flows from the influence zone and into the conduit. In both embodiments, an air mover such as a blower or vacuum pump establishes a significant pressure differential in the influence zone.

[0016] In a preferred embodiment, a blower pressurizes the system conduits relative to atmosphere, and air flows through the influence zone, the adjacent soil, and ultimately back to atmosphere. If the influence zone is saturated, the pressure of air causes the water in the influence zone to physically move away from the conduit and the zone is de-saturated. When not fully saturated, the pressure of air flow causes physical gas exchange, to make the composition of gas in the influence zone more near that of atmosphere. In another embodiment, air flows similarly, but from an unpressurized conduit to an auxiliary pipe which is buried in the soil and maintained at below atmospheric pressure. In another embodiment, and air mover lowers the pressure of gas in the conduit and air flows from atmosphere, through the soil and influence zone and into the conduit. In another embodiment of the invention, air flows from a pressurized auxiliary pipe buried in the soil adjacent the trench, and into a conduit vented to atmosphere. In another embodiment, air is introduced into the bottom of the leaching trench by a pipe diffuser or by pipes which run lengthwise within the trench. In preferred practice, for a wastewater system embodying typical conventional soils, the differential air pressure between the conduit and atmosphere is at least 2.5 mm, preferably about 7.5 cm or more, water column, in order to produce a desired level of biochemically significant flow through the influence zone. When, in preferred embodiments, air is flowed from a conduit into the influence zone, the pressure in the soil of the influence zone, about 15 cm from the inner boundary of the influence zone, will be at least about 0.03 mm water column, more preferably at least 0.8 mm.

[0017] In further accord with the preferred process of the invention, the influence zone of a deteriorated system is substantially anaerobic in character, and flowing of air or active gas causes the change so that it becomes predominantly aerobic. In still further accord with the invention process, the quantity of air or other gas which is flowed into the influence zone provides oxygen substantially in excess of the stoichiometric quantity which is required for oxidation of the oxidizable constituents in the wastewater, as such constituents are typically determined by measurement of Oxygen Demand, in particular Biological Oxygen Demand (BOD). Optionally, a gas or liquid substance is added to the air to enhance biochemical activity.

[0018] In still another embodiment, the auxiliary pipe is buried under the trench or is within the trench. A membrane is optionally placed within or on the soil, to control the direction in which air travels from or to the soil surface. The air flow of the invention may be maintained continuously or intermittently, with and without simultaneous flow of wastewater in a preferred, practice of the invention, by means of a control system. To ensure good functioning of a system, a low volume of air is continuously flowed into the wastewater system and the air moves through the influence zone contemporaneously with wastewater.

[0019] In further accord with the invention, apparatus for treating wastewater is comprised of a primary unit, such as a septic tank or other kind of reactor for primary processing of the wastewater, a leach field, for receiving wastewater effluent of the primary unit, where the leach field is comprised of a trench in the soil, a conduit within the trench, and soil adjacent the trench comprising an influence zone; and a means, such as a blower or vacuum pump, for producing a pressure differential between the conduit and the adjacent soil, where the pressure differential is significant enough to effect a physical change, such as forcing water from the influence zone soil, or to effect biochemically significant change in the biochemistry of the influence zone.

[0020] In preferred apparatus embodiments, there is a means, such as a mechanical check valve or a water trap, in the pipeline of the wastewater system, so the effect of applied pressure or vacuum is limited to localized parts of the wastewater system. In one instance, there is a check valve in the distribution pipe, which runs from a septic tank or the like to a distribution box, or other distribution piping, and air pressure is injected at one or more selected points in the distribution piping or conduits. In another instance, there is a check valve in the wastewater line downstream of the stack vent and upstream of the point at which pressurized air is injected. In still other embodiments, pressure or vacuum is applied to the wastewater line running into the septic tank and a check or other valve is present upstream of the point of connection to the wastewater line of the blower or vacuum pump source of differential pressure. In another embodiment, a blower is in the stack vent of the system. Use of the valve means enables use of the system for processing wastewater simultaneously with use of air flow. A check valve bypass line, temporary storage reservoir and pump are optionally in the wastewater line to further aid in the objective of continuous use. The duration or periods during which air is flowed is optionally controlled by a control system which senses the composition or pressure of gas or liquid in vicinity of the influence zone or elsewhere in the wastewater system.

[0021] In a further embodiment of the method of the invention, the apparatus is configured so that there is a void space above the soil surface of the influence zone, through which wastewater flows downwardly. For instance, the void might be the space within an arch shape chamber or the spaces amongst stones within a trench. The water is first flowed in sufficient quantity and rate until it accumulates as a pond above the soil surface. The air pressure is applied to the void, to give impetus to the natural flow of water. Then, the application of air pressure is ceased, and either the water flow is resumed right away, or there is a period of quite time which is substantially longer than the time of applying air pressure. The cycle may be repeated continuously. The process saves energy by decreasing the duration of air mover operation.

[0022] Preferably, the amount of air or other gas provides a quantity of air per unit time of system operation which quanity is many times greater, e.g., 10 to 25 times or more, than the quantity which theoretically would satisfy the biological oxygen demand of the wastewater.

[0023] The invention is effective in improving the operation of leach fields in a cost-effective way. Leach field performance and biochemistry are improved and maintained through use of the system. The invention can be applied to existing installations and new installations. It is quite useful for rejuvenating the function of a system which has been either under-designed or over-used.

[0024] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of best mode embodiments and accompanying drawings.

[0040] Reference should be made to the Background section hereof for a description of various components, processes and environment which relate to both typical septic tank wastewater systems and the invention. FIG. 1 illustrates components of a typical septic tank wastewater system, familiar in the prior art. Wastewater flows through wastewater line 18 from a dwelling or other source to a septic tank 20. Wastewater flows from tank 20, through a distribution system. In particular, the wastewater flows through distribution pipeline 22, to distribution box 30; and, then through further distribution pipes 56 to the leach field 40. FIG. 1 shows a composite of two familiar types of leaching field construction. FIGS. 2 and 3 respectively show elevation cross section views through each of the two types. In two branches, or laterals, of the leach field 40 which is shown, a trench 35 is filled with crushed or naturally small stones 36 typically about 2.5 cm in dimension, (hereafter referred to as “stone”); and, a perforated pipe 32 distributes wastewater along the trench. The wastewater issuing from the pipe 32 may be temporarily stored in the voids within the stone; and, as indicated by the arrows 54, it then flows into the soil 38 (also called soil profile) beneath the surface 42 of the earth through the bottom and or side walls of the trench. A typical biomat 44 forms on and within the surface of the trench. Additional biomat, not shown, will typically be present on the stones in the trench, and may be present to such an extent as to run like a membrane across stones which lie at the perforations or openings of the pipe. While typical, a biomat will not necessarily be present in a wastewater system.

[0041] Referring to FIG. 3, in one branch, or lateral, of the leach field 40, the trench is filled by a string of interconnected molded arch shaped plastic leaching chambers 34, such as Infiltrator® chambers. In many respects the functioning of the chamber system is similar to the stone filled trench and thus will not be repeated. Wastewater flows into the hollow interior of the chamber from pipe 56, where it may be stored temporarily, and it then flows into the soil, out the open bottom wall of the trench and through perforations in the sidewall, as indicated by arrows 54. The chamber defines and maintains the trench, and provides the conduit means for distributing wastewater along the trench. Sometimes, a pipe, which is a continuation of line 56, runs along the length or a chamber interior peak to abet distribution of wastewater within a string of interconnected chambers. The side walls of trench 35 are defined by the chamber walls and the bottom is parallel to the base of the chamber. Sometimes, the trench is made larger than the chamber, and crushed stone surrounds the chamber. From the foregoing it will be appreciated that both the chambers 34 and perforated pipes and conduits which both convey and disperse wastewater at different locations.

[0042] In both types of popular leach field constructions the infiltration rate into soil is often limited at least in part by the presence of one or more biomat layers. And, in both types there will be within the soil adjacent to the trench an influence zone 50, having a nominal outer boundary 52, where the wastewater is (or should be, in a properly functioning wastewater system) largely renovated, or biochemically converted into a more environmentally benign form, prior to re-introduction into the ground water. The geometric definition of the influence zone is somewhat arbitrary and it can vary in dimension with time, as will be appreciated from the discussion in the Background and herein. In the drawings here, the outer boundary 52 is imaginary and not intended to represent any limiting dimension or proportion. While the invention is described in terms of soil of the earth, it will be understood that such term comprehends installations comprised in whole or part of artificial porous material such as sand and gravel and other particulate media.

[0043] The distribution portion of the FIG. 1 system includes a typical distribution box 30, which is intended to balance flow between the branches of the system. In some septic systems there is only one branch, or several legs of the leach field are connected in series, in which case the distribution box is omitted, and the invention will be understood as being applicable in obvious ways to such systems. There are other commercial and non-commercial devices which are useful as conduits, in substitution of the stone filled trenches and chambers in leach fields; for instance, concrete galleries, leaching pits, also called “dry wells”, and drip line, as used in irrigation systems. In the generality of the invention herein, chambers and stone-trench pipes, dry wells, galleries, and all other substitutional devices—whether surrounded with small stone or buried directly in soil, are all considered conduits of fluid, within a context that some serve other functions as well. For example, cesspools and injection wells are also considered as comprising conduits and as functioning in part at least as leach fields. Cesspools are devices that function as both a primary unit and a leaching unit. Typically, raw sewage is introduced directly from the source, and as biochemical action takes place in the pool, the wastewater percolates into the surrounding soil. In injection wells, wastewater is pumped into the earth through a vertical pipe so it disperses into material beneath or around the pipe. Most of the rest of the description here uses the example of leaching chambers.

[0044] A main part of the invention comprises pressurizing or evacuating leach field conduits, relative to atmosphere. Pressurizing is described first. FIG. 4 shows a semi-schematic side elevation view of a gravity flow septic tank system, which is an improvement on the essential system shown in FIG. 1. A wastewater line 18 runs downwardly from building 60 to septic tank 20, for primary processing. Wastewater from an unshown sanitary appliance or other source is introduced into inlet end 62 of the wastewater line. Gases, which come up the wastewater line from the septic tank, are vented through stack vent 58 which protrudes above the building roof When wastewater 64 flows into the septic tank, baffles 66 prevent floating solids from moving out of the tank an into the first part of the distribution subsystem, effluent exit pipe 22. The primarily treated wastewater flows from the septic tank, through check valve 70, into distribution box 30, and then into leach field 40 which is comprised of several parallel strings of leaching chambers 34, one of which strings is shown. Blower 71 causes pressurized air, drawn through air inlet pipe 72 to flow down air pipeline 68 and into the distribution box. Check valve 74 protects the blower from back flow of wastewater and smelly gas when it is not in operation. An odor-absorbing charcoal filter may optionally be used with or without the check valve.

[0045] When the blower 71 is activated, the air pressure increases in the leach field. Check valve 70 prevents flow backward up effluent line 22. Referring to FIG. 4, with reference to FIG. 2 and FIG. 3, the pressure within the leaching chambers is raised sufficiently to cause flow of air through the biomat 44, if any, and through the portions of the trench walls to which air passing through the chamber walls has access. The air flows into the influence zone 50. From there, the air may flow in various directions within the soil 38, ultimately escaping from soil surface 42 to atmosphere. Thus air flows serially through the conduit interior, the trench wall, and the influence zone, in the same direction as wastewater flows. Should there be a substantial quantity of water in the soil of the influence zone when the blower is started, the pressure of the air will desirably hasten the flow of water out of the influence zone. Some air may move directly upwardly from the trench interior to the surface. If the soil above the trench is saturated, as it may be in a severely failed leaching system, upward flow should be helpful in restoring the system. In experiments, it has been found that upward air flow is not a particular problem. In many systems, the soil tends to be compacted above the trench, which inhibits such flow. As discussed below, a barrier on or in the soil above a leach field trench will tend to inhibit upward flow.

[0046] The air flow into the conduit is maintained for a desired time, according to the result sought and obtained. After a time, air flow may be ceased, so the system resumes its normal operation or, the system may be used during the time of air flow, as described below. Typically, a low power centrifugal blower is sufficient for producing the pressure differential which induces flow through the soil. The means for providing pressurized air will depend on the resistance of the air flow path. A regenerative type blower is used if a relatively high pressure is needed. Alternately, other means for flowing air may be used, such as blowers, fans, compressors, etc., according to the air moving performance required and the price of the component. Unless stated otherwise, all pressures stated herein are gage pressures, i.e., relative to atmosphere.

[0047] Biomat is normally anaerobic. To the extent such is present, air flow will tend to make it at least temporarily aerobic, thus engendering desirably different biological or chemical reactions which alter and reduce the biomat. In one mode of the invention, air is forced into the influence zone and beyond, along the same general flow paths which wastewater has. Obviously, to the extent the influence zone and adjacent soil are saturated, there will be a physical/hydraulic effect, as forcing air into them will tend to unsaturate them. To the extent the influence zone is not appropriately aerobic, the flow of air tends to make it so, both by physical displacement and by desirable biochemical activity. If there is an accumulation of oxygen-demanding constituents within the influence zone and surrounding soil, they will be desirably oxidized and reduced in amount. To the extent there is excess undesirable gas in the soil, such as methane, carbon dioxide and hydrogen sulfide, they will be incorporated into the air stream and carried away, e.g., by diffusion in accord with the law of partial pressures; and, the changed environment will selectively affect their production.

[0048] Thus, in synopsis, the invention process maintains or improves, as the case may be, the function of a leach field by (a) a physical (mechanical, pneumatic, hydraulic) effect; and (b) a chemical and or biological effect, which for simplicity if referred herein to as a biochemical effect, or in related fashion, to a change in biochemistry.

[0049] With respect to the biochemical effect, it is generally accepted that in a major way the efficacy of the influence zone in treating wastewater depends on the soil within the zone comprising a predominantly aerobic environment, as such is contrasted with an anaerobic environment. In a typical failed system where for one reason or another the influence zone is anaerobic, the capability for treating wastewater is as little as one-tenth that of a desirable aerobic environment which enables aerobic metabolism and oxidation. An aerobic environment is said to exist when the oxygen tension is sufficient to predominantly sustain the growth of aerobic bacterium. See Chapter 10 of the textbook of R. Atlas, “Microbiology, Fundamentals and Applications,” MacMillan Publishing Co., New York (1984). For this description, an aerobic soil is one which is macroscopically aerobic, e.g., the preponderance of the volume of the soil of interest has oxygen tension sufficient for sustaining predominantly aerobic bacterium. An anaerobic soil in this description is one which is not aerobic. Nonetheless, within a mass of aerobic soil, such as the influence zone, the environment may vary from point to point, and anaerobic conditions can exist within a predominantly aerobic environment. For example, anaerobic micro-pores may be present within an array of aerobic macro-pores.

[0050] The gross environment in the influence zone can be looked at in another way, according to principles of stoichiometry. As is conventional in wastewater flow analysis, wastewater can be characterized according to Oxygen Demand. Oxygen Demand is a characterization of how much oxygen is needed to effectively treat the oxidizable constituents in the wastewater to make them environmentally benign. Oxygen Demand is usually divided into two constituents, namely Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). COD is commonly measured by the so-called Hach Method 8000. For wastewater systems associated with habitations, BOD is the commonly used parameter of interest. It is commonly measured in accord with United States Environmental Protection Agency Standard 405.1. Stoichiometry, as applied here to oxygen demand, refers to the chemical balance between an oxidizable material and oxidizer, in this instance, within the influence zone. If the influence zone receives a quantity of air which is stoichiometric, it means the quantity is exactly that which is necessary for complete theoretical oxidization of the oxidizable matter in the zone. In real situations, there are imperfect mixing and other influences, and to achieve full oxidation, some excess air (oxygen) is required. Thus, from this viewpoint, the desired aerobic influence zone will be characterized by oxygen-bearing gas flow over time, e.g., hours, which is in substantial excess of stoichiometry for the oxidizable constituents of the wastewater entering the zone over the same time. The excess stoichiometric air flow can derive from artificial means, such as through use of the invention, by natural means such as migration through permeable soil, or by a combination of those. As an example of such practice, for the system referred to in connection with Table 1, a continuous flow of 280 l/minute of injected air provides 50-100 times the stoichiometric quantity for a typical about 1200 l/day wastewater stream.

[0051] Accordingly, in the invention, air is characterized as an active gas. It maintains or alters the composition of constituents and biochemistry of the influence zone. It contains oxygen and enables oxidation. Accordingly, other active gases may be used with or in substitution of air. For instance, oxygen or ozone enriched air may be used for better activity, and an oxygen-helium mixture may be used for a combination of greater fluidity and activity. Optionally, liquid or gaseous additives, such as reactive substances like hydrogen peroxide, surfactants, and the like, may be introduced into the air stream in line 68 from additive system 76, by pumping or other obvious means. See FIG. 4. Alternatively, they may be introduced separately into the leach field conduits at the distribution box, distribution pipes 56, or chambers. Included within the effects of such additives will be to promote wetting, to act as solvents, and to increase oxidization or to promote other biochemical activity beyond what air induces.

[0052] Thus, in one aspect of the invention, an influence zone will be saturated with water and the pressure and flow within the soil of the influence zone is sufficient to unsaturate it. In another aspect of the invention, an influence zone will initially have a composition that is deficient in oxygen, being biochemically significantly less than the volume percent oxygen in air, and it will be changed so it approximates the composition of air. For instance, a starting composition of 14-19% oxygen will be changed to 20-21%, at a point about 15 cm from the inner boundary of the influence zone. In another aspect of the invention, the influence zone and surrounding soil will have constituents, in addition to oxygen, which are substantially different from the composition of air. For instance, carbon dioxide may exceed 2%, methane 1% and hydrogen sulfide 0.005%. Use of the invention will change such values in the direction of the composition of air.

[0053] In the invention process there will be a flow into the influence zone of a quantity of gas which is effective. That is, the quantity will produce part or all of the above described desired physical or chemical change. Of course, the quantity or total mass of gas put into the influence zone is a product of the flow rate and time. The flow rate of the air within the soil profile will depend on the permeability of the particular soil profile and the applied differential pressure. Generally, it is an object to keep the air mover small for purchase and operating cost and noise reasons. Thus, in one mode an air mover is run continuously to pressurize the leach field, with or without simultaneous use for wastewater. In another mode the air mover is run intermittently, either during periods of wastewater use, or in periods of non-use.

[0054] In a primary aspect of the invention, the amount of gas which is flowed into the influence zone is biochemically effective. Put another way, the air flow is sufficient in quantity to alter the biochemical conditions in the influence zone from those conditions which would exist in the absence of flowing, to a degree which is significant to the treatment of wastewater and soil. What is meant by biochemical or biochemistry is described above. Another primary aspect of the invention is to physically move water from the influence zone and to replace it by air, to a degree sufficient to create an aerobic region in part or all of the influence zone and or to increase the permeability of the zone. What is an effective amount can vary with the starting condition in the influence zone. Thus, an effective amount of gas will change the zone so that treatment of wastewater, which simultaneously or subsequently enters the zone, will be significantly better treated. Thus, one or more of the following parameters will be meaningfully and significantly affected: the quantity of wastewater (e.g., volume and oxygen demand) which can be handled in a unit volume of influence zone will be increased; the quality of the effluent from the influence zone will be higher in the context of common environmental standards for underground water, the permeability of the zone will be increased; the pressure necessary to cause a unit of gas flow through the zone will drop; the pressure gradient within the zone for a given flow of gas will drop; the zone will change in character from anaerobic to aerobic; the quantity of water resident in the zone at any given instant will decrease; the fraction of oxygen in the zone will increase; and, the fraction of gases associated with anaerobic activity, such as carbon dioxide, methane and hydrogen sulfide, will be decreased in the zone.

[0055] It follows from the foregoing, that to get a significant quantity of air flow there must be a commensurate significant pressure differential applied across the influence zone. Usually, it is desired to effect the change in a short period—a matter of hours or days, which may lead the user toward higher pressures. A physical movement of water within the zone usually occurs in a matter or minutes; and, a significant change in biochemistry will usually occur in hours, and will approach equilibrium in hours or days. The rate at which the beneficial effects are achieved depend on the air pressure/flow, the initial and changed conditions of the influence zone (e.g. permeability, temperature, etc.), any continuing introduction of new wastewater having oxygen-demand, and so forth.

[0056] During use, the soil profile around a properly functioning wastewater system leach field will be moist, inasmuch as tension and capillarity will cause water to be retained. Moisture provides a soil profile with a flow resistance significantly greater than that of dry soil. Thus, simply flowing air down a conduit of a moist leach field, where the end of the conduit is vented to atmosphere, does not achieve the objects and benefits of the invention. While the environment within the conduit may be affected, there will be insufficient static pressure within the conduit to induce flow into the surrounding soil, owing to the resistance of moist soil profile and biomat if any to flow. Obviously, if the influence zone is saturated, the chance for flow is even more remote. Experiments with perforated pipe and leaching chambers buried in sand more permeable than typical soil, using fans like those suggested by the prior art, fail to induce biochemically or physically effective flow into the influence zone. To obtain flow from a pressurized conduit that will significantly affect the influence zone of a typical leaching field within a matter of hours or days, in the desired ways described herein, the conduit static pressure will preferably be in the range 1-100 cm water column. Obviously, when a liquid is to be pushed from a saturated zone, the initial pressure required is higher than if there is no saturation. If there is saturation, and it is alleviated by air treatment, the necessary pressure drops. Likewise, when the biochemistry is changed, soil permeability tends to be increased, and the necessary pressure drops. Experiments to date for a typical system, described in connection with Table 1 below, have been conducted. Relatively low pressures and per unit area mass flow rates occur in the influence zone and surrounding soil. For instance, a blower providing 290-1400 l/minute at 2.5-100 cm water column may effectively be used in a leach field having an estimated trench area sidewall and bottom area of 74 m². A nominal flow rate soil immediately adjacent the trench wall will be of the order of 0.26-1.3 m³/m²/minute. After treatment, a Table 1 kind of system might be biochemically maintained by a continuous flow from a lower capacity blower, for instance one with capacity of 1700 l/minute operating at static pressure as low as about 2.5 mm water column. In most applications, on-going or maintenance pressures will probably be higher, so it will be desirable to have a blower that produces at least about 7.5 cm water column pressure.

[0057] Differential pressure may be applied for many hours, extending into days or continuous operation, to achieve the desired effects. Thus, it can be very difficult to detect flow and pressure gradients, the presence of which is inferred and implicit because there is a measurable change in the gas content of the soil during use of the invention, as shown by the example below. The efficacy of the invention is often best determined by desirable change in, or maintenance of, gas content of the soil. In particular, oxygen is made high; carbon dioxide, methane, hydrogen sulfide and nitrogen compounds are made or kept low; all in the direction of the content of atmospheric air or in a biochemical direction which is better than air for aerobic bacterium.

[0058] The apparatus in FIG. 4 may be modified somewhat to make an alternate embodiment in which a vacuum, instead of pressure, is applied to the distribution box, distribution pipes or chambers of the leach field. Check valve 70A is replaced by a manual or motorized valve. The valve is closed during vacuum operation—assuming the vacuum level is such that it would unacceptably draw the contents from the septic tank. Any liquid drawn toward the vacuum pump is appropriately trapped by common means, and the liquid is re-introduced into the wastewater line or septic tank. When a vacuum is applied, air is drawn from the surface of the earth, through the soil, the influence zone, any biomat, and into the chamber, in ways which will be appreciated from the other description here. The air drawn from the system by the vacuum pump may be exhausted to atmosphere. Thus, there is serial flow which is the reverse of that which is described when the conduit is pressurized. The leaching system cannot be used for processing wastewater from the septic tank during the time it is placed under vacuum.

[0059] With reference to FIG. 5, in another embodiment of the invention, one or more pressure-vacuum perforated pipes 72 (called auxiliary pipes hereafter) are installed, either as original equipment or as retrofit, in proximity to, and parallel to, the chamber-filled trenches 35. Air is either drawn from the pipes 72 or forced into the pipes 72, so that the leach field is beneficially affected. In one mode, pipes 72 are connected to a vacuum pump and gas is pulled from the soil. Appropriate common traps and trap emptying means are used to handle any wastewater that is drawn into the auxiliary pipes and to the pump, and the trapped water is recycled to the septic tank or leach field. The evacuation of pipes 72 pulls air from the chambers to the extent they are in communication with atmosphere by means of vents in the distribution box (e.g., in substitution of line 68) or chambers, or through the septic tank and stack vent 58, consistent with other discussion herein.

[0060] When auxiliary pipes are vented, air will be drawn from the earth surface 42 downwardly through the soil 38, as indicated by arrows in the Figure, according to the vacuum level, the permeability of the soil and conditions inside the chambers. Preferably, there is significant flow through the chambers and trench walls, to replicate to a degree the effect achieved when the chambers are pressurized, as described above in connection with FIG. 4. To encourage such, auxiliary pipes 72 are placed sufficiently close to the trenches/chambers, and deep enough, to create a pressure gradient across the biomat if any and within the influence zone. As shown in FIG. 5, a membrane, such as plastic sheet 96, is optionally placed in the soil profile above the pipes 72 to limit downward flow of air in between the chambers. When the chambers are not vented to enable air flow directly from atmosphere to the chamber interior, the membrane tends to cause the air to flow from surface 42 towards, into and through, the chambers. In one alternative, the membrane is a layer of relatively impermeable natural material, laid within the indigenous soil. In another alternative, the membrane is placed on the earth surface 42. Preferably, the membrane runs substantially horizontally; in other modes, the membrane may run vertically or at an angle with or without a horizontal membrane. In still another alternative, the surface of the earth is compacted or otherwise treated to lessen permeability and to thereby functionally create a membrane.

[0061] FIG. 6 shows a stone filled trench and pipe 72, together with a coordinated graph showing the estimated nominal variation in pressure P versus depth from the surface. The pipe 72 is beneath and parallel to a stone filled trench 35 and evacuated. A sub-atmospheric pressure gradient is established along the arbitrary vertical reference line D, as indicated by the dashed line P on the right side of the Figure. The pressure gradient induces the air flow indicated by the arrows on the left side part of the Figure. The air flows through the soil above the trench, more freely through the spaces amongst the stones, and then vertically down through the soil beneath the trench, and into the pipe. Corresponding pressure gradients and flows will be inferred for chambers having evacuated auxiliary pipes directly below.

[0062] In an alternate embodiment of use construction in accord with FIG. 6, auxiliary pipes 72 are pressurized to thus create a positive pressure gradient and related flow within the soil. This will be essentially the inverse of the phenomena just described for evacuated pipes. Thus, the pressure in the soil in vicinity of the trench, chamber, or pipe, as the case may be, will be supra-atmospheric and air will flow into the pipe and upwardly through the soil profile to the chamber and or soil surface. When the pipes 72 are beneath the influence zone, pressure or upward air flow may also retard the downward flow and escape of wastewater to ground water, which could be either desirable or undesirable depending on reactions in the influence zone.

[0063] FIG. 7 is a fragmentary picture which will be understood from its analogy to FIG. 5. FIG. 7 shows one of a plurality of spaced apart vertical pipes 78, which is positioned adjacent one or more of the trenches, along the length of the trench, in place of the horizontal pipes 72. The pipes 78 have perforated lower ends 79, to enable air to flow into and from the soil. As for other like Figures, the flow of gases, according to whether the pipe is pressurized or evacuated, is indicated by the double-headed arrows. Pipes 78 may alternatively be angled downwardly so the lower pipe ends are positioned beneath the trench, rather than adjacent to it as shown. Thus, the term “vertical pipe” will encompass pipes which are generally running in an upward direction, whether true vertical or at an angle to such, in which case they are only generally vertical. The advantage of the use of vertical injection or suction pipes is one of easy retrofit. Combinations of horizontal vertical (and angled) pipes and means for interconnecting them to the source of pressure or vacuum will present themselves. The horizontal or vertical aeration pipes shown in FIG. 5 or 7, and the diffusers of FIGS. 14 and 15 discussed below, may be installed proximate or within individual trenches, or portions thereof, rather than generally throughout an entire leaching system, to cope with a localized problem.

[0064] The operation of the FIG. 5 embodiment, as well as operation of other embodiments of the invention, may be controlled as indicated by the schematic control diagram portion of FIG. 5. The composition and or pressure of gas in the influence zone or soil adjacent the zone is sensed using a gas sensor comprised of a probe 74 and an analyzer 76. The analyzer provides a first signal to controller unit 94 which compares the first signal to a desired reference point (characteristic of a desired reference gas composition or pressure) and provides a second signal to the air mover 71, to thereby modulate flow of air to or from pipe 68 and pipes 72, according to the difference between the first signal and the reference point. The air flow may be modulated by stopping and starting the air mover, or by increasing or decreasing the pressure. Pressure is alternatively measured within the conduit or something connected to it, because as indicated, a drop in static pressure for a given flow is a measure of leach field improvement. The reference composition is selected in accord with the teachings herein. For instance, the reference composition may be such as 20% oxygen, or less than 1% carbon dioxide. The reference pressure is selected in relationship to other parameters which measure the performance of the influence zone, and based on accumulated experience with like wastewater systems.

[0065] FIG. 14 is a view like FIG. 3, showing a chamber having a oblong cross section pipe 94, called here a diffuser, buried at shallow depth beneath the soil at the base of the chamber. Diffuser 94 may be alternatively laid on the soil at the base of the chamber. Diffuser 94 is connected to a source of pressurized air, so pressurized air is forced into the chamber in an evenly distributed way, to achieve the desired effect which has been described in connection with FIG. 5-7. FIG. 15 is like FIG. 14 and FIG. 3, showing a perforated wastewater pipe 32 in a stone filled trench 35. Running along the length of the bottom of the trench are two small diameter auxiliary pipes 96 carrying pressurized air. In another alternative, the pipes 96 are shallowly placed in the soil 38 beneath the trench, in manner analogous to FIG. 14.

[0066] Referring again to the FIG. 4, there is a further embodiment which is useful, when the leach field chambers are pressurized. It comprises using the characteristic of the septic tank outlet to provide the means which prevents the back flow of air toward the wastewater line 18, instead of employing check valve 70. For instance, when the septic tank outlet is an elbow, the head or water in the septic tank may resist back flow pressures of 2-100 cm water column. If the septic tank by design permits through-flow of air, such as when a tee fitting is used instead of an elbow, then the septic tank outlet or inlet may be modified by blocking off the openings which permit air passage. When these circumstances exist, the flow of wastewater into and out of the septic tank may be maintained during air pressurization of the leach field, assuming the resultant increase in head at the inlet end of the septic tank is tolerable. This will be further understood by the discussion which applies to the use of a water trap valve upstream or downstream of the septic tank, below.

[0067] Another alternative system design is as follows. It is very common that a septic tank will enable unimpeded flow-through of gases, from outlet to inlet. When such is the case, the location of the check valve 70 shown in FIG. 4 for the pressurized leach field may be changed. It may be moved to wastewater line 18, upstream of the septic tank, and the pressurized air line 68 may be connected between the valve and the septic tank. FIG. 8 shows such a construction in semi-schematic fashion, comprising check valve 80 in wastewater line 18. Of course, the septic tank cannot leak excessively out the top if this option is to be practical. A check valve which is useful in this embodiment and others described is a common horizontal flap type check valve sold in connection with PVC wastewater pipe of 7.5-10 cm diameter. Manually or power actuated mechanical valves referred to herein are likewise commercially available.

[0068] The side elevation semi-schematic view of FIG. 9 shows another way for preventing back flow which can be used in substitution of a check valve, namely J-trap 82. The trap type is familiar for toilets and other plumbing appliances. Sometimes, it is referred to as a U-trap. Generally, the J- or U-type of traps, and any other functional equivalent, is referred to hereafter as a water trap. As illustrated in FIG. 9, wastewater water is retained in the base of the J-shape contour of line 18. The asymmetrical nature of the J shape means that there is more resistance to upstream flow of a fluid, compared to downstream flow. Thus the J-trap is functions analogously to a check valve, in impeding flow of air and water in the downstream direction more than in the other direction. Obviously, the back flow resistance can be overcome by raising the pressure sufficiently high, but so can a mechanical check valve be overcome by sufficiently high pressure. Obviously, the depth of the J can be used to set the head to which the downstream part of the septic system can be subjected. In the generality of the invention involving pressurization, the valve in the wastewater line functions to limit upstream movement and the valve may be a mechanical or water trap. It will be appreciated that in certain circumstances a water trap valve may be functionally sufficient and thus useful on the downstream side of the septic tank.

[0069] FIG. 10 is a semi-schematic elevation view of a variation on FIG. 8. The primary wastewater line 18 is fitted with valve 80A. Bypass wastewater line 86, with associated bypass check valve 82, provides a bypass around the valve 80A. Valve 80A is kept open during periods when there is no air pressurization of the system, i.e., during use of the wastewater system as it takes place in absence of use of the invention. Thus, with appropriate pipeline angling and positioning, wastewater will not consequentially pass through the check valve. This eliminates possible noise that might be heard by occupants of the building, arising from operation of check valve 82 as wastewater passes through.

[0070] FIG. 11 is a view like FIG. 10. It shows a plumbing arrangement which is useful when the pressure in the leach field, due to air flowing down feed pipe 68, is more than the head of wastewater coming down the wastewater pipe can overcome. In such instance, the valve 80B in the main wastewater line 18A is closed. This causes wastewater coming down the wastewater line 18 to accumulate in reservoir 84. By means of a reservoir level sensor system, the pump 87 forces wastewater through line 86 and forces it into the septic tank. Check valve 82B prevents any back flow when the pump is not operating.

[0071] FIG. 12 is like FIG. 10 and shows still another plumbing variation. Flow to reservoir 84 along bypass line 86 is controlled by valve 82C. When the valve 82C is closed, wastewater flows along the main wastewater line through check valve 82. When the valve 82C is opened, wastewater preferentially flows down line 86 into reservoir 84 from whence, according to signals from unshown sensors, the pump 87C is activated to force wastewater down line 86C to overcome the air pressure in the wastewater system. The apparatuses described in connection with FIG. 10-12 may be alternatively installed in exit line 22 or distribution line 56.

[0072] The elevation view of FIG. 13 shows how a centrifugal type blower 71 can be placed in the stack vent line 58, when J-trap 92 (or a functional substitute, e.g., a plurality of appliances having integral traps), or a mechanical valve (e.g., on-off or flap-check), is present to prevent back flow of pressurized air, up the wastewater line 18 into building 60. When actuated, the blower forces air downwardly in the stack, and into the septic tank. When not actuated, the blower allows wastewater decomposition gases to escape up the stack. Particularly for retrofit purposes, the blower may be placed at the upper end of the stack vent, exterior of the building.

[0073] For original installation or retrofit purposes, a kit of components, which cooperate when installed, may be provided, e.g., to an installer of wastewater treatment system owner. The kit may comprise all or part of the following: a means for connecting an air flow supply to a wastewater or a distribution line, a flow checking means, an air mover, and a control system.

[0074] In the most likely way in which the invention will be used, the leach field conduits will not be vented to atmosphere along their lengths or at their ends farthest from the septic tank. However, the invention can be applied to such kinds of vented conduits. When the chambers are pressurized, there is an obvious disadvantage in that the air mover has to have capacity sufficient to achieve the necessary static pressure while accommodating the loss of air out the vent. Alternately, the vents are fitted with valves. And, as has been mentioned, in various modes of the invention, e.g. when the conduits have subatmospheric pressure, it is contemplated that the conduits will be vented through the septic tank and stack vent. For such modes, other vents into the conduits can have a positive or neutral effect.

[0075] The following is an example of the practice of the invention. Pressurized air was applied to the wastewater line of a long-functioning single family dwelling septic tank type system. Table 1 shows selected representative measurements of composition of soil profile gas content and pressure over time. The septic tank permitted free flow-through of air, thus the pressure in the leach field conduits was the same as at the wastewater line. The leach field was comprised of two 30 m serially connected lengths of perforated pipe in stone filled trench. The indigenous soil was comprised of silt, with some clay and a trace of fine sand and gravel. The soil temperature adjacent the trenches was about 13 degrees C. The arrangement was like that shown in FIG. 8, with a check valve 80 in the wastewater line. The air mover was a 93 w, 1700 l/minute blower. Soil gas composition was measured using a commercial gas analyzer and a probe which inhibits infiltration of surface air. See U.S. Pat. No. 6,018,909. Pressure was measured with the same kind of probe. The applied positive pressure at the wastewater line was continuous and ended at the time shown for the readings. It gradually declined as the flow increased with the passage of time. Wastewater use was continued during the test, with at an estimated 1200 l/day input. Readings S7S and S7D were taken in the influence zone, respectively at depths of 30 and 75 cm from the earth surface at the end of a 30 m first trench length. Reading S1S was taken at the inlet end of the same first trench. All readings were taken at 15-30 cm from the inner boundary of the influence zone, that is, from the outer surface of the stone filled trench, which was not well defined in the long-used system. Generally, subsurface pressure measurements can be quite variable, which is in large measure believed due to local variations in the essential soil. For instance, a horizontal worm hole may have a significant local effect. The data in Table 1 represent typical or average data. They show how the influence zone of the soil was desirably changed in the aerobic direction over time and how unwanted constituents were decreased. The methane reading for S6D is apparently anomalous.

[0076]

TABLE 1
Soil gas composition change as a function of time for a pressurized septic system.
	Conduit	Volume Percent	Parts Per Million	Influence
Probe	Time	Pressure		Carbon		Hydrogen	Zone Pressure
Location	Hr	mm H2O	Oxygen	Dioxide	Methane	Sulfide	mm H2O
S7S	0	0	20.5	0.84	10	0	0
	16	200	20.1	1.18	5	0	0.050
	408	120	20.9	0.1	0	0	0.025
S6D	0	0	19.1	2.00	0	0	0
	16	200	19.6	1.56	0	0	0.065
	408	120	20.4	0.98	15	0	0.050
S1S	0	0	14.3	>5.00	0	0	0
	16	200	16.5	4.38	0	0	0.025
	408	120	20.9	0.16	0	0	0.075

[0077] In other variations of the invention, air may be applied continuously or discontinuously. Generally, it is desirable to limit the amount of time in which air is flowed, to decrease the cost of operating the blower or other air mover. Often, the dynamics of the system will permit discontinuous air flow. In some instances, water is flowed in alternation with the flow of air or other active gas. In many instances, such as in the case of a domestic dwelling, wastewater flow varies over the course of the day, and the time of flowing air may be varied in coordination with such. For example, a blower may be run for one hour of presumed non-use, during the nighttime hours. For another example, a blower may be run for two equally spaced apart half hour periods each day. As in the examples, the time for airflow may be relatively small as a fraction of the total time of a wastewater flow day. In another application, a dosing pump or other means periodically delivers water to the leach field from a septic tank or other sump.

[0078] In a further embodiment of the invention, the leach field is periodically dosed with wastewater, sufficient to create ponding, or a standing water level, on top of the soil of the influence zone. For example, the inside of the conduit of FIG. 3 is supplied with water at a rate and quantity such that it accumulates within the chamber on the underlying influence zone. For another example, the trench of FIG. 2 is likewise supplied for a first period of time by flow through the perforated pipe, so that water accumulates to a certain level within the broken stone of the trench. When the pond is created, the flow of water is ceased. Then after some delay to allow natural percolation, or alternatively without delay, air pressure is applied to the conduit. The pressure is maintained for a certain second time period, which may be a matter of a quarter hour, or more or less. The air pressure time is sufficient to force the wastewater to flow through the influence zone downwardly and otherwise, and to cause air to flow into the influence zone. Then the air pressure is ceased and the system is allowed to remain quiet for a third period of time, which period of time is substantially greater, for example, two times or more greater, than the time during which air was flowed. During the third period of time, the oxygen which is delivered by the airflow has a biochemically beneficial effect within the influence zone, as has been previously described. Alternatively, the third time period may be essentially zero. Monitoring of system performance and the character of water exiting the influence zone will help determine what the third time period should be. Finally, the cycle is repeated again with another dosing of wastewater, nominally like the first cycle.

[0079] In one mode of alternating air flow with water flow, engineering judgment is used to determine the times of flows and a simple cycle timer is used to control the system. In another mode, the wastewater system is fitted with a more elaborate control system comprising part or all of the following. A sensor, such as a float or other commercial liquid level sensor is installed in the void space. A signal from the sensor stops the inflow of water by the dosing pump, when the water reaches a predetermined level. Immediately, or after a timer-controlled period, the blower is energized by the control system, to apply the pressurized air. Then, an oxygen sensor embedded in the influence zone is used to determine when air has sufficiently flowed into the influence zone, and to stop the flow of air. A timer then determines when dosing is restarted. The timing and duration of dosing will obviously also be a function of the wastewater which is flowed into the system.

[0080] In further practice of the invention, air is provided in an amount which is greatly in excess of the calculated amount which would be theoretically necessary to satisfy the oxygen demand of the waste water. Typically, such demand for a domestic dwelling will be primarily BOD. The following is an illustration of such a situation. A system has a flow of about 1140 l/day (about 300 gal/day) and a BOD of about 150 mg/l, resulting in a calculated oxygen demand of about 0.17 kg/day. Air is flowed to the leach field at about 2.5 cm (1 inch) water column pressure and about 285 l/min (10 cubic feet per minute) for one hour per day, so that the total air flow is about 17,000 l. Thus, there is a flow of about 4.5 kg of atmospheric oxygen through the system. And, the weight flow of oxygen gas was about 26 times the weight of BOD in the waste water. The BOD of the wastewater exiting the influence zone is tested and found to be about 1 to 5 mg/l.

[0081] In another example, a commercial establishment has a flow of about 1500 l/day (350-400 gal/day,) of about 1000 mg/l BOD to a leach field. Air is supplied on a continuous basis at the rate of about 2850 l/min (100 cubic feet per minute). Thus the input oxygen is about 1080 kg/day while the oxygen demand of the wastewater is about 2.6 kg/day. So the flow of oxygen in the air is about 400 times the amount of oxygen demanded by the wastewater. In the generality of the invention, the airflow per unit time provides at least several times the theoretical oxygen demand of wastewater per unit time. Typically, a unit of time that is convenient is a 24 hour day.

[0082] While the septic tank system designs described herein are most widely used in connection with dwellings, the invention will be useful in wastewater systems which process wastewater streams from industrial, agricultural, food processing, commercial, etc., operations, which wastewater streams present the same disposal problems as domestic wastewater from habitations. In a septic tank system, the septic tank is a unit for primary processing or treatment of wastewater and the leach field provides secondary processing or treatment of the wastewater, as such terms are well understood in the art. Septic tanks are commonly known to comprise predominantly anaerobic environments. The invention will also be useful with leach fields which derive wastewater from other sources than primary units. Other devices, both aerobic and anaerobic, may be substituted for septic tanks as the primary (treatment) unit. For instance, an aerobic reactor which agitates the wastewater with air may be used.

[0083] Although this invention has been shown and described with respect to a preferred embodiment, it will be understood by those skilled in this art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.