GB1586685A

GB1586685A - Process and apparatus for treating and purifying waste water

Info

Publication number: GB1586685A
Application number: GB29247/77A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-07-16
Filing date: 1977-07-12
Publication date: 1981-03-25
Also published as: DE2731887A1; FR2358361A1; DE2731887C3; JPS5623672B2; FR2358361B1; DE2731887B2; MY8500803A; JPS5331355A

Description

(54) PROCESS AND APPARATUS FOR TREATING AND PURIFYING WASTE WATER (71) We, TADASHI NIIMI and MASAAKI NIIMI, Japanese Citizens of 88-104, Sagamiono-Kodan-Jutaku, 3897, Kamitsuruma, Sagamihara-shi, Kanagawa-ken, Japan, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a process and apparatus for treating and purifying waste water.

Water pollution in rivers and coastal waters has been in progress on a worldwide scale by discharge of waste water to them. In every country physical, chemical and biological steps have been integrated in many ways to purify waste water, stipulating water quality standards in rivers and coastal waters to which treated waste water has been discharged. However, removal of eutrophic salts including nitrates and phosphates has not been enough, failing to completely eliminate pollutants.

In order to purify. waste water, it is believed that the following three fundamental measures should be adopted. The first is to discharge as small an amount as possible of pollutants included in the waste water from the processes of industrial production and our daily life. The second concerns recovery of pollutants in treatment processes. The last is treatment of waste water and sludges in soils in situ, without discharging them to rivers and coastal waters.

The process and apparatus of this invention aim to achieve the above-mentioned second and third measures in an integrated and rational way.

According to the invention there is provided a process for treating and purifying waste water, which comprises the following steps: (a) feeding influent waste water to a waste water treatment tank laid under the ground in situ.

(b) causing the surface of the waste water to rise and fall in a water level fluctuation region defined between the top and bottom an upwardly convex net lying over a bed of sand and/or gravel in the tank which sand and/ or gravel is supported on an apertured grid above the base of the tank, the sand and/or gravel and the net being covered by an overlying layer of soil, (c) and discharging a part of the waste water to outside of said tank, a pro portion of the water leaving the tank by percolation through the soil by capillary action and the remainder continuing to rise and fall in the tank.

The invention further provides an apparatus for treating and purifying waste water according to the above process, which comprises an elongated waste water treatment tank laid under the ground in situ, a base conduit extending along the bottom of said tank overlaid by a perforated grid on which is supported a bed of sand and/or gravel occupying the tank above the grid, an upwardly convex net lying over said bed with an overlying layer of soil covering the net, and inlet means for feeding influent waste water to one end of the tank and delivering it into said base conduit and said bed.

Particular techniques according to the invention will now be described by way of example and with reference to the accompanying drawings, in which : - Fig. 1 shows a model of waste water treatment in situ by a prior art capillary siphoning purification method.

Fig. 2 shows filtration rates by capillary siphoning in different soil bed materials.

Fig. 3(A) shows the principle of rapid decomposition and purification of waste water and sludges by a capillary siphoning purification method according to the invention.

Fig. 3(B) is an enlarged view of part of Figure 3(A).

Figs. 3(C) and 3(D) are diagrams further illustrating the method of Figs. 3(A) and 3(B).

Fig. 4 shows a ground plan of an apparatus of this invention.

Fig. 5 shows a vertical section of an apparatus of this invention.

Fig. 6 shows a cross section along the line X-X in Fig. 4.

Fig. 7 shows a cross section along the line Y-Y in Fig. 4.

Fig. 8 shows a cross section along the line ZZ in Fig. 4.

In the conventional land treatment of municipal waste water, gravity percolation and filtration has been mainly applied for agriculture in many countries. This invention not only makes in a scientific way the utmost use of capillary siphoning phenomena in soils with complicated soil water movement, but also makes the best use of microbial activities in soils, so that groundwater contamination can be eliminated, decomposition of sludges promoted in soils and offensive odour removed.

Consequently, this invention will be extreme applicable to a country like Japan where pollution control policy aims to prevent discharge of waste water to closed water basins such as lakes and rivers, and the United States where a no-discharge policy will be realized in future to all kinds of navigable waters, because by this invention waste water will be purified in situ where the treatment tank is set up.

Basic Difference between Conventional I,and Treatment and this Invention: The conventional land treatment of municipal waste water and sludges has started from digging a shallow hole in the ground in-M which sewage is discharged.

Then the hole is covered with soil in which all the wastes disappear almost completely by decomposition in less than a year. The idea developed from this prototype enables one to purify treated waste water by percolation in soils, whether or not by sprayirrigation, inflltration-percolation or overland flow, and also to purify untreated waste water with an offensive odour by discharging it into an underdrain intermittently. However, in the treatment of waste sludges new processes have not been found that exhibit greater technical advantages than the conventional process in which sludges are covered with soils. Intermittent incorporation ofi increased amounts of waste water into soils will be apt to contaminate groundwater when the ground has vertical holes to transport water, or when the soils contain gravels or volcanic ashes which are too porous, because the retention time and quantity of even treated waste water in the upper soil layer will then be not sufficient for complete purification by micro-organisms in the soil. In other words, the purification capacity of upper soils will not be realized enough in this case.

We believe that- the regimes of soil water where micro-organisms are most active are either in capillary water supported between soil particles or in free water moving between capillary pores or in water contained in microscopic pores of crumbled soil structures. When waste water is treated continuously by capillary siphoning in soils, much attention must be paid to prevent the contamination of underground water caused by percolated effluent.

For this purpose new techniques have been developed to overcome the problems.

The invention relies on an infiltrationpercolation method combined with the phenomenon of capillary siphoning.

In this method, all floors and side-walls of underground treatment tanks are waterproofed. However, some effluent can flow out laterally over the tops of the walls at a depth in the range of tens of centimeters below the land surface by the capillary siphoning phenomenon. In other words, the upper soil layers, which is tens of centimeters deep, that covers the underground tank is utilized as a culture medium for micro-organisms. In this way, waste water can be completely purified in a simple installation that makes the best use of the phenomena of capillary siphoning and mirco-organism activity. Although this waste water treatment is so simple, it can exceed any conventional complicated techniques of engineering in sewage treatment in the aspect of efficiency.

Purification of Waste Water by Capillary Siphoning: In Figure 1, the soil surface is represented at a and the highest level of waste water in a waste water treatment tank is represented at b. When the vertical distance Hs between the highest level b and the lowest level of waste water b' is the water level fluctuation range within which the level of waste water moves up and down.

A tank side-wall is represented at c, the top of which is at d. The vertical distance between a and d is the depth H3 of the soil layer above the tank; this layer is tens of centimeters deep.

Through the soil layer represented by H3 + Hl, waste water percolates by capillary action and a siphon is established as indicated by the arrows, the down leg: of the siphon outside the tank terminating in a screen e at the level of the tank bottom.

H1 is the vertical distance between b and d and is the minimum height that capillary water is required to rise. H is the vertical distance between b and e and represents the gravity effects in the siphon when the waste water is at its highest level b. The surface of the groundwater below the tank is represented at f.

In experiments conducted on waste water filtration by capillary siphoning, the verti cal distance H2 is usually more than 20 cm.

H1 is a very important design value, determined by a function having amongst its variables the rate of inflow of waste water and the filtration rate by capillary siphoning that occurs in the specific soil present. In ordinary soils, rates of filtra tion, in meters/day, by capillary siphoning can be represented by the curves shown in Figure 2, when H2 exceeds 20 cm.

Japanese Patent Nos. 582612 and 590927 deal with the case where H2 is decreased as much as possible, and percolation in the capillary siphonage system is increased, to give discharge of purified water at the screen e. However, in this invention the screen e is eliminated.

Rapid Separated Sludge Decomposition: Separated sludge will be produced in a solids settling tank, solids/liquid separator, aeration tank, sludge storage tank, intermediate sedimentation tank, and so forth.

Two kinds of sludges have to be dealt with. One is settling sludge and the other is sludge that floats to the surface of water by reason of the gases it contains. In the conventional digestion and decomposition of sludges, anaerobic decomposition has been employed. Such sludge as floats to the surface of water usually sinks to the bottom after having discharged its gases.

However, in the course of time some sludge on the surface of water can become dry and change to a more or less permanently floating scum. This will make decomposition by micro-organisms difficult, and eventually an accumulated layer of thick, hard scum will build up, preventing adequate waste water treatment. As a counter-measure, frequent agitation of the tank contents has been employed to stir up floating sludge and prevent scum formation, and also withdrawal of the sludge has been tried. However, no-one has ever come up with an idea for achieving rapid sludge decomposition on the surface of waste water.

To deal with the problem, the waste water is introduced into a gravel layer covered by a soil layer, as shown in Figure 3(A). In Figures 3(A) and 3(B) the soil surface is represented by L, a treatment tank has side-walls F which can be of concrete, poly-vinyl plastics film or other suitable material,- and a durable chemical net N divides gravel in the tank from the overlying soil layers. Large stones S in the gravel in the tank have a diameter of about 10 cm, which avoids clogging even with very dense sludges. A deposit of sludge 0 sinks to the bottom of the tank, while floating sludge O' attaches to the stones in the gravel.

Fluctuations of waste water level between the highest water level HWL and the lowest water level LWL take place not only due to variations in inflow of waste water, and removal of water by capillary siphoning as indicated by the arrows, but also by the evaporation, traspiration and capillary percolation that take place all the time.

Sludge that rises up to the surface of the waste water.due to the gases it contains will not settle out immediately after hav ing discharged its gases, but will either attach to the surface of stones in the gravel or stay in small pore spaces of the gravel layer.

In a treatment tank in use, fluctuation of the waste water level between HWL and LWL can be expected to take place several times a day.

The intermittent supply of water and air to the surface of the gravel between HWL and LWL has surprisingly been found to promote the decomposition of the cellulose fibres of toilet paper, which is the main material included in municipal waste water sludges that is hard to decompose. However, it is known that a wooden telephone pole, which is not decayed easily either above the ground level or in water, tends to rot at the region just under, i.e. ten centimeters or so under, the ground surface.

Prevention of Secondary Environmental Pollutions: As is illustrated in Figures 3(C) and 3(D) waste water will rise into the overlying soil by capillary motion in the direction indicated by the solid arrows. In Figure 3(C), there are represented at g gases such as ammonia, methane, hydrogen sulphide and so forth, that will be produced by decay and fermentation and will rise with the waste water, or air that may be present due to aeration, or other gases containing oxygen. The broken line arrow indicates the upward diffusion of gas bubbles. Figure 3(D) illustrates a model enlarged to show the three soil phases and the habitats of micro-organisms. So represents individual soil particles which, in the actual soil, will be part of the general crumbled structure. Small gaps between soil particles will be filled with water, while in larger gaps the bounding soil particles will carry capillary water films m with an air void v between. In an ordinary soil layer, the water can be interlinked continuously in this manner for distances up to about two hundred centimeters, in both horizontal and vertical directions from the surface of the influent waste water that fluctuates between HWL nd LWL. The quantity of water withdrawn by evapotranspiration and capillary siphoning will be continuously replaced from the surface of the influent waste water.

The gases g diffuse through the gaps v between the capillary water films m in a tortuous manner toward the atmosphere as shown by the curved line arrow. Gases which are soluble in water or easily absorbed by soil particles, such as oxygen, ammonia, methane and hydrogen-sulphide, will be absorbed or adsorbed by the capil lary water films or the surfaces of the soil particles to the point of saturation in each case. These gases that are absorbed and adsorbed will then be decomposed by aerobic micro-organisms J or anaerobic micro-organisms K living in the capillary water films, being transformed to odour less materials such as nitrate, sulphate and so forth. These materials being, of course, soluble in water, they will be washed down by rainfall, so that the soil particles will not remain saturated for long with these materials but a more or less continuous process of odorous gas removal will occur.

Rising gases in a body of water appear as bubbles at the water surface and the bursting of the bubbles causes water drop lets to be impelled into the atmosphere.

Water droplets with a diameter of more than 30 microns will fall back immediately on to the surface of the water but other smaller ones will be dispersed, transporting with them such pathogens as the water may contain. In the present system, how ever, water droplets impelled from the surface of the waste water will collide with the boundaries a' of the soil particles surrounded by capillary water films and will thus be removed in an efficient way by the covering soil.

At the same time, plants T, P growing in the soil (Figs. 3(A) and 3(B)) will absorb inorganic substances and water from the soil through their roots, releasing the water into the atmosphere by transpiration.

Apparatus according to this Invention for Treating Waste Water: A specific embodiment of apparatus is now described with reference to Figures 4 to 8. From a pipe 1 municipal waste water is discharged into an inlet region 2 of an inlet chamber 3 in one end of a long comparatively narrow waste water treatment tank which is laid under - the ground and has side-walls 4 and- a base 4'. -The inlet chamber 3 has a cover 2'. On both sides of the base are supports 5 holding a grid of spaced transverse battens 6, 6, 6...

Above the battens is a sand-gravel layer S overled by an aerated soil layer s. At the boundary of the two layers is an upwardly convex net N, through which the level of waste water can fluctuate up and down between HWL and LWL, forming a water level fluctuation region.

Influent from the bottom of the inlet chamber 2 flows along base conduit 9 under the battens 6 and passes up through the openings 6', 6', 6' ... between the battens 6, 6, 6..., or directly from the inlet chamber 3 through side-holes 3', first into the sand-gravel layer S and then up to the bottom edge 8 of the convex net N. With the normally expected daytime rate of flow of influent, the surface level of the waste water will continue to rise to HWL near the top 7 of the convex net with further supply of influent. This is determined by appropriately choosing such measures as, for instance, the height and diameter of an overflow drain-pipe 11 which communi cates with an outlet region 2 of an outlet chamber 3 similar to the inlet chamber 3 but at the opposite end of the tank and likewise having side-holes 3'. Below the bottom of the convex net capillary siphoning does not occur, which determines LWL.

Between the bottom and top of this convex net is the water level fluctuation region.

Waste water rising above LWL will perco late as capillary water into the upper aerated soil layer s as well as into the main body of soil s' outside the tank side-walls 4, as represented by arrows in Figures 6 and 7. Therefore, if the flow of influent is interrupted the water level will gradu ally fall to LWL. The influent will not be suppied at a constant rate. For instance, less waste water will come in at night.

However, any waste water in the water level fluctuation region will be losing water at- all times by capillarity and the capillary siphoning phenomenon even when there is no suppy of fresh influent, resulting in the withdrawal by evaporation, transpiration and capillary movement of treated waste water. In this way, the level of waste water being treated will tend to descend from HWL to LWL. When the flow rate of influent again becomes greater than the rate of loss of treated waste water due to the above effects, the water level will tend to rise from LWL to HWL. In this way the waste water level will go up and down in the water level fluctuation region depending on the rate of influent.

Reduction in the amount of influent reaching the base conduit 9, or an excessive amount of water in the upper aerated soil layer s, can -cause water in the aerated soil to descend by gravity together with the micro-organisms it contains through the sand-gravel layer S, and down into the base conduit 9 through openings 6'.

Renewed flow of influent at a high rate will cause the waste water to rise again to the net N through the sand-gravel layer S, and water will again escape as vapour to the atmosphere, as represented by the top arrow in Figure 6, according to the conditions, after having passed through the net as capillary water to the upper soil layer. These processes will be repeated over and over again, with progressive purification of the residual waste water by the micro-organisms in the soil layer and those carried down into the sand-gravel layer and tht base conduit 9. Finally, a portion of the residual water arriving at the discharge end of, the tank, completely treated, will flow away through the outlet pipe to a near-by river, or the sea or to an underground discharge point.

As -an example of actual design of -the apparatus, the distance from the ground surface to the top of the tank side-walls 4is.usually-30 cm., the width between the side-walls is about 60 cm., the 'distance between the top of the side-walls and the bottom of the battens 6 is 9.70 cm., the depth of the base conduit is 9.30 cm., and the width of the base conduit 40 cm.

The vertical distance between the top of the convex net N and the bottom of it, which includes the water level fluctuation region, will usually-be 10 cm. The length of the tank will depend upon the required waste water treatment capacity, in that the longer the tank, the greater the treatment capacity.

A conventional waste water treatment tank, particularly a settling tank, ranges from one to four.meters deep, its shape being !round or square. :Such design is to make it .easy to remove sludge in maintenance. In the case of-the-ong and shallow base conduit employed in the present apparatus, there is no-dead water and the rising and falling of the waste water effici- ently decreases environmental pollutions including odour and pathogens, as follows: a) Percolaion of treated waste water can be greatly increased by capillary siphoning.

b) Such buoyant materials as oils and fats, sludges containing bubbles and so forth will be detained in the pore spaces of the sand-gravel layer S above the base conduit 9. Therefore, it will easily be understood that the longer conduit will increase efficiency in separation.

c) As was depicted in Figure 3(A), decomposition of sludge is carried out in the region H, between HWL and LWL, so that decomposition capacity will be greater in a less deep tank with a greater plan area. As will be described hereinafter, the sand-gravel layer S, where the largest number of earthworms and micro-organisms will be able to live, is made the utmost use of to decompose sludge.

d) Complete treatment in situ of waste waters from a variety of sources can be realized within the side-walls 4 and in the soil above and around the tank. There fore ditches and so fdrth employed in conventional waste water treatment are unnecessary, nor does the apparatus re quire much expense for installation and maintenance. Treatment of waste water ranging in amount from 10 1/day to 40 1/day can be realized in ordinary soil by a single apparatus of the kind des cribed.

As an optional provision, a perforated pipe 10 made of plastics can be set in the base conduit 9 and provided with forced draught from an air-blower P, driven by an electric motor M. The air escapes through the small perforations in the pipe 10 to aerate the waste water. As an ex ample, the perforated pipe 10 installed in base conduit 9 can be. made of polyethylene with a diameter of 65 mm: If de sired, earthworms and soil organisms.such as cellulose-decomposing - micro-organisms can be added to the water level fluctuation region by closing.

By setting up a perforated blast pipe 10 in the base conduit 9 through which forced draught is provided by the power driven air-blower P, - the decomposition of the waste water and sludge by micro-organisms in the base conduit 9 and soil organisms in the water level fluctuation region covered by the convex net N is more highly activated. In this way, the role s of rthe base conduit 9 becomes similar to that - of a conyentional aeration tank, but with the special benefits of the present invention as already discussed.

To sum up, this apparatus can purify waste water and digest sludges, removing environmental pollutions such as offensive odour and pathogens, by making use of a process in which in a continuous way, the fluid level fluctuates repeatedly in a liquid gas region in a tank, utilising the activities of useful soil organisms.

As to the application of this invention, it cannot be employed for the treatment of influent containing heavy metals. However, compared with conventional land dispersal- by sprinkler, we have discovered that the process according to this invention makes it possible to treat influent possessing a considerable amount of pathogen and offensive odour. Therefore, domestic effluent including refuses, and waste water from the food-processing industry, are materials that can be treated at very low capital, operation and maintenance costs.

WHAT WE CLAIM IS:- 1. A process for treating and purifying waste water, which comprises the following steps:

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

S, and water will again escape as vapour to the atmosphere, as represented by the top arrow in Figure 6, according to the conditions, after having passed through the net as capillary water to the upper soil layer. These processes will be repeated over and over again, with progressive purification of the residual waste water by the micro-organisms in the soil layer and those carried down into the sand-gravel layer and tht base conduit 9. Finally, a portion of the residual water arriving at the discharge end of, the tank, completely treated, will flow away through the outlet pipe to a near-by river, or the sea or to an underground discharge point.

As -an example of actual design of -the apparatus, the distance from the ground surface to the top of the tank side-walls 4is.usually-30 cm., the width between the side-walls is about 60 cm., the 'distance between the top of the side-walls and the bottom of the battens 6 is 9.70 cm., the depth of the base conduit is 9.30 cm., and the width of the base conduit 40 cm.

The vertical distance between the top of the convex net N and the bottom of it, which includes the water level fluctuation region, will usually-be 10 cm. The length of the tank will depend upon the required waste water treatment capacity, in that the longer the tank, the greater the treatment capacity.

A conventional waste water treatment tank, particularly a settling tank, ranges from one to four.meters deep, its shape being !round or square. :Such design is to make it .easy to remove sludge in maintenance. In the case of-the-ong and shallow base conduit employed in the present apparatus, there is no-dead water and the rising and falling of the waste water effici- ently decreases environmental pollutions including odour and pathogens, as follows: a) Percolaion of treated waste water can be greatly increased by capillary siphoning.

b) Such buoyant materials as oils and fats, sludges containing bubbles and so forth will be detained in the pore spaces of the sand-gravel layer S above the base conduit 9. Therefore, it will easily be understood that the longer conduit will increase efficiency in separation.

c) As was depicted in Figure 3(A), decomposition of sludge is carried out in the region H, between HWL and LWL, so that decomposition capacity will be greater in a less deep tank with a greater plan area. As will be described hereinafter, the sand-gravel layer S, where the largest number of earthworms and micro-organisms will be able to live, is made the utmost use of to decompose sludge.

d) Complete treatment in situ of waste waters from a variety of sources can be realized within the side-walls 4 and in the soil above and around the tank. There fore ditches and so fdrth employed in conventional waste water treatment are unnecessary, nor does the apparatus re quire much expense for installation and maintenance. Treatment of waste water ranging in amount from 10 1/day to 40 1/day can be realized in ordinary soil by a single apparatus of the kind des cribed.

As an optional provision, a perforated pipe 10 made of plastics can be set in the base conduit 9 and provided with forced draught from an air-blower P, driven by an electric motor M. The air escapes through the small perforations in the pipe 10 to aerate the waste water. As an ex ample, the perforated pipe 10 installed in base conduit 9 can be. made of polyethylene with a diameter of 65 mm: If de sired, earthworms and soil organisms.such as cellulose-decomposing - micro-organisms can be added to the water level fluctuation region by closing.

By setting up a perforated blast pipe 10 in the base conduit 9 through which forced draught is provided by the power driven air-blower P, - the decomposition of the waste water and sludge by micro-organisms in the base conduit 9 and soil organisms in the water level fluctuation region covered by the convex net N is more highly activated. In this way, the role s of rthe base conduit 9 becomes similar to that - of a conyentional aeration tank, but with the special benefits of the present invention as already discussed.

To sum up, this apparatus can purify waste water and digest sludges, removing environmental pollutions such as offensive odour and pathogens, by making use of a process in which in a continuous way, the fluid level fluctuates repeatedly in a liquid gas region in a tank, utilising the activities of useful soil organisms.

As to the application of this invention, it cannot be employed for the treatment of influent containing heavy metals. However, compared with conventional land dispersal- by sprinkler, we have discovered that the process according to this invention makes it possible to treat influent possessing a considerable amount of pathogen and offensive odour. Therefore, domestic effluent including refuses, and waste water from the food-processing industry, are materials that can be treated at very low capital, operation and maintenance costs.

WHAT WE CLAIM IS:- 1. A process for treating and purifying waste water, which comprises the following steps:

(a) feeding influent waste water to a waste water treatinent tank laid under the ground in .situ.

(b) causing the surface of the waste water to rise and fall in water level fluctuation region defined between the top and bottom of an upwardly con vex net iying over -a- bed of sand and/or gravel in the tank which sand and/or gravel is supported on an apertured grid above the base of the tank, the sand and/or gravel and the net being covered by an overlying layer of soil.

(c) and discharging a part of the waste water to outside of said tank, a pro portion of the water "leaving the, tank by percolation through the soil by capillary action and the remainder continuing to n.e and fall in the tank.
2. A process according to claim 1, wherein said influent waste water enters an inlet chamber whence it flows to said sand and/ or graves bed either along a base conduit under said, grid and up through the apertures in the grid, òr through side-holes of the inlet chamber.
3. A process according to claim 1 or claim 2, wherein the height and diameter of a tank outflòw pipe are chosen such that said surface of the waste water is caused to rise substantially to the top of said convex net when the influent is arriving at the normally expected day time rate.
4. A process according to claim 1 or claim 2 or claim 3, wherein soil organisms such as- earthworms and/or microbes Such as cellulose-decomposing bacteria and fungi are added to said fluctuation region.
5. A process according to any one of the preceding claims, wherein air is supplied by an air-blower delivering to a perforated - pipe near the bottom of the tank to aerate the waste water in the tank.
6. An apparatus for treating and purifying waste water according to the process of claim 1, which comprises an elongated Waste water treatment tank laid under the ground in situ, a base conduit extending along the bottom of said tank overlaid by a perforated grid on. which is supported a bed of sand and/or gravel occupying the tank above the grid, an up wardly convex net lying over said bed with an overlying layer of soil covering the net, and inlet means for feeding influent waste water to one end of the tank and delivering 'it into said base conduit and said bed.
7. An apparatus according to claim 6, wherein earthworms arid/or microbes such as cellulose-decomposing bacteria and fungi are added to said water level fluctuation region.
8. An apparatus according to claim 6 or claim 7, further comprising a perforated aeration pipe set in the base conduit and an air blOwer to provide forced draught-to said pipe.
9. A process for treating and purifying waste water substantially as described herein.
10. Apparatus for treating and purifying waste water substantially as described with reference to Figures 4 to 8 of the åccomparíying drawings