The Purification of Polluted Waters in Permaculture

Permaculture Designers Manual



Section 7.5 –

The Purification of Polluted Waters in Permaculture

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The only long-term insurance of good water supply to a settlement is by rigorous control of a forested catchment, including a total ban on biocides and metallic processing.

As there are few such clean areas left in the world, house roof tanks must do for the foreseeable future.

The 30-40 additives commonly introduced into water supplies are often pollutants in themselves to that increasingly sensitive sector of society developing allergies to any type of modern pollutant.


These additives represent the end point of the technological fix:


pollution is “fixed” by further pollution.”

Herein, I will stress the biological treatment of common contaminants; the only water safe for us is also safe for other living things.

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For millennia we have existed on water supplies containing healthy plants and fish and if we keep natural waters free of feces and industrial contaminants, we can continue to do so.

This is not so much a matter of water treatment, as the prevention of polluting activities.

However, for cities and towns, sewage and storm water supplies must continue to represent a “disposal problem“.

As the wastewaters or upstream settlements are the drinking waters of downstream areas, our duty is to release from any settlement only water of sufficiently good quality to be safely useable by others.

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The problem contaminants most likely to affect drinking water are:

TURBIDITY: silt and fine particles suspended in the water.

BACTERIAL or ORGANIC pollution from sewage and as decay products, e.g. E. coli, disease organisms and viral or protozoan pathogens, parasitic worm eggs and so on.

METALLIC POLLUTANTS such as chromium, cadmium, lead, mercury.

BIOCIOES e.g. Aldrin; Dieldrin; 2, 4-D; 2, 4, 5-T; dioxin; PCB. etc (organophosphates, halogenated hydrocarbons).

EXCESSIVE FERTILISER especially  nitrogenous compounds, phosphates, sodium and potassium salts.

ACIDS or acid-forming compounds (a pH less than 5.5 increases metallic pollution).


Many of these factors interact.

Acid rain dissolves out of rocks and soil poisonous forms of aluminum, mercury, lead, cadmium, and selenium or other metals such as copper, nickel and lead from drinking tanks, tea urns and hot water tanks.

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Organisms may convert inorganic mercury to organic forms (as happened in Minamata, Japan) which are readily absorbed by the body.

Sewage in water aids such conversion to biologically active metals.

Mercuric fungicide dressing on seeds has not only caused direct poisoning of people who have eaten the seed, but also poisons the soil.

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Excessive artificial fertilizer increases aquatic biological activity, which results in further uptake of metals in acidic waters and so on. In biocides, Aldrin prevents DDT being excreted; the combination is deadly (one can buy this mix in Australia and the third world, or farmers will achieve it by successive sprays.

DDT is a stable residual poison co-distilling with water, so that distillation will not help.

An additional threat to public health comes from the many miles of asbestos pipe used in public water supply systems; there is a definite threat of both stomach and bladder cancer from asbestos particles in water supplies.

Ferric acid aluminum sulfate, salt and lime are all added to water to cause fine particles to flocculate and settle out as clay.

In England, as pH increases due to acid rain and in fact wherever acid rain occurs, aluminum goes into solution, and with lead and cadmium may bind to protein in vegetables and meal, especially those boiled or steamed.

Even if salt is added to cooking water to decrease these effects, levels far exceeding the 30mg/1 allowable for those with kidney problems are experienced.

Cooking may increase the water content of metals by a factor of 5 due to this protein binding and make the metals easier to assimilate in the body.

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Cooking acidic substances in aluminum pots simply worsens the problem. Aluminum from acidic rain leaching is now thought to be a major cause of tree and lake death.

Ferric sulfate may be safer to use, especially if water is initially or reasonably alkaline. Obviously, these effects need more study and any inorganic salt or metallic salt deserves very cautious use.



AERATION (oxygenation) by wind, mechanical aeration or by increasing turbulence in flow. Aeration is also achieved by trickle columns and vegetation, phytoplankton or injected air.

SETTLING: spreading flow in still-water ponds or rush beds to allow particles  to fall out, filter out or flocculate.

SKIMMING and SIEVING to remove large organic particles.

FILTRATION via sand beds or charcoal-fiber columns, soils, the roots of aquatic plants.

COAGULATION or FLOCCULATION by using chemical additives (lime, salt, ferric sulfates ) or organic (bacterial) gels.

BIOLOGICAL REMOVAL by bacteria, phytoplankton and higher plants.

pH ADJUSTMENT by adding calcium (as lime) or sulfur compounds as needed .



A classical and widely-used filter is sand.

Britain and many cities use sand filters followed by chlorination to clean settled and treated raw sewage water sedimentation.

Filtration by slow drip through 1.2 m (4 feet) of sand (the top-half is fine sand with the bottom-half being coarse sand) is used even in temporary rural camps for water filtration (Figure 7-32).

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For cities, fixed sand beds with brick bases are used, the top 1 cm (0.5 inch) or so of sand periodically swept, removed and dried or roasted to remove organic particles before the sand is returned.

Activated charcoal often from bones or plants such as willow or coconut husks, is also used as a fine filter in homes and where purity is of the essence.

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Fine dripstone (fine-pored stone) is used in water cleaners and coolers to supply cool water in homes.

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Trickle filters through sand and gravel columns actually feed resident bacteria which remove the surplus nutrient.

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In less polluted environments, a similar task is carried out by freshwater mussels.

Carbon is essential for the removal of nitrogen or for its conversion by bacteria to the gross composition C5H7NO2 and it is generally added as carbohydrate, which can be liquids such as methanol, ethanol, or acetic acids, many or all of them derived from plant residues.

This is a bit like “adding a little wine to the water” to encourage the bacteria to work.

Surplus nitrogen is released by bacteria to air. Unless bacteria are encouraged and allowed to work, nitrates move easily through sub-soils in which no plants or bacteria can live and can emerge in wells and streams.

In ponds intended for drinking, light exclusion and surface water stabilization reduce both turbidity and thus algae, to a minimal quantity.

The stabilization of banks by grasses and dump plants helps considerably.

Pond surface stabilizers are water lilies, Azolla and water hyacinth. Bank stabilizers are Juncus, Scirpus, various grasses and clovers, Phyla nodosa (Lippia) and bamboo and pampas grass clumps.

With turbidity much reduced, filtration loads are likewise reduced. Liming will further reduce turbidity if pH is 6.0 or less.Insert Here

This is as simple as placing crushed marble or limestone as a layer in a tank or casting burnt lime over a pond before filling and (if necessary) after filling.

Crushed shells or even whole shells in water tanks and ponds have the same effect. Lime flocculates particles, causing them to settle out of the water.

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There are several techniques for filtration, some or all of which can be used in series.

First, trickle filters of loose pebbles (2.5-10 cm) can be used to form an active bacterial surface layer to absorb nutrients, and then a sand filter can be used to absorb bacterial pollution. Water rising through a sand column is fairly clear.

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The shells of water mussels can be substituted for pebbles and the living mussels in the pond or tank not only monitor acidity (dy1ng at pH 5.5 or thereabouts) but filter, individually, up to 100 1/day.

Digesting bacteria and depositing wastes in the mud base. Mussels and crayfish are not only susceptible to low pH but are also very sensitive to biocides such as Dieldrin, so that their living presence is a constant monitor on life-threatening pollution.

Water, now fairly clean, can be passed through a bed of watercress to remove dyes and nitrates and the cress cut and fed to animals or dried and burnt to ash.

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As a final process, the water can be trickled through a column (a concrete pipe on end) of active carbon (10%) and silicon dioxide (90%), otherwise known as burnt rice, oat, or wheat husks (bio-char).

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The results should be clear, sparkling, safe water to drink. No machinery is involved if the system is laid out down slope to permit gravity flow

Lime (freshly burnt) is often used to remove phosphorus and sludge’s in a primary settling lagoon and then water is passed to a trickle tower for ammonia removal by bacteria.

In towers, of course, the bacteria are not further consumed, but in open lagoons a normal food cycle takes place, with myriad insect larvae and filter-feeders removing bacteria and frogs, fish and waterfowl eating the insects.

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In small towns, the water can be passed from filter towers to sewage lagoons, which in fact may become rich waterfowl and forest sanctuaries. It can then be routed to field crop such as forest or pasture and to crops to be distilled or burnt, which does not directly re-enter the food chain.

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Sewage Treatment Using Natural Processes

Raw sewage is a mixture of nutrients, elements, heavy metals, and carbon compounds; it also contains quite dangerous levels of bacteria, viruses and intestinal worm eggs.

A typical analysis is given in Table 7.3.

Units are as mg/1; samples are of 30% industrial, 60% domestic wastes at Werribee, Victoria, Australia (Hussainey, Melbourne Metropolitan Water Board Pubs., 1978).

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Melbourne is a city of 2,700,000 people and its sewage lagoons cover 1,500 ha (3,700 acres).

Thus, there is one hectare of pond (in total) to 1,800 inhabitants (or about 1 acre for 820 people). In the ponds, raw sewage is run into about 724 ha (1,790 acres), where it settles out.

Each of these primary settling ponds rarely exceeds 7 ha, (17 acre) in area; so, about 100 ponds receive and settle all raw sewage. Scaled down, this means 1 ha (2.5 acres) of settling pond to 3,800 people.

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All these settling ponds are anaerobic and give off biogas, a mixture of methane (CH4), carbon dioxide (CO2) and ammonia gas (NH3), with traces of nitrous sulfide or marsh gas (NO2).

Biogas is, of course, a useful fuel gas for engines or a cooking gas for homes.

However, it is also a gaseous component of the atmosphere that is creating the “greenhouse effect” and thus should be used, not released to air.

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The next set of ponds is facultative (as described below) and the last set aerobic.

These, in total, slightly exceed the area of the anaerobic or settling ponds. Most are 7-10 ha (17-25 acres) in size.

Ponds can be built (as they are at Werribee) to fall by gravity now from one to the other.

In the first series of (settling) ponds, the sludge creates an ANAEROBIC condition. In the next series of ponds, some sludge passes over and becomes anaerobic at the pond base, while the surface water in the pond (due to wind or algae) is AEROBIC (oxygen-producing).

The final series of ponds is totally aerobic. Thus, from intake to outlet, we have the terms:

ANAEROBIC, or methane-producing (digester ponds).
FACULTATIVE, or part methane, part oxygen producing.
AEROBIC or oxygen-producing ponds.

Ponds at Werribee are only an average of 1m (3 feet) deep. Deeper and the sludge breakdown and wind aeration effects are less.

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One thousand townspeople and their associated industries, therefore, need as little as 270 square meters of settling pond 1m deep.

We could, in fact, achieve this as a “long” pond (or series of ponds) 3m wide x 90m long, or 3 side-by-side ponds 30m long and 3m wide or any such combination.

We can halve the length by doubling the depth to 2 m. and get a pond 3 m x 45 m long; or treble the depth and condense the pond area to a 3m deep x 3m wide x 30m long “digester pond“.

Such a long and narrow pond is easily made totally anaerobic by fitting water seals and a weighted cover over the top (which can be of plastic, metal, butyl rubber, or fiberglass). Note that for these deeper digester ponds we would need to artificially agitate the sludge (using pumped biogas to stir it), otherwise it settles and becomes inactive (Figure 7.33).

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Sludge is “active“‘ only in contact with the semi-liquid inputs of the sewer; thus when we stir up the sludge, the better we break down the sewage to biogas.

Another (critical) benefit in sealed and agitated digesters is that no scum forms on the pond surface, which can slow the break down process further and cause an acid condition.

Of the total dissolved solids (or influent) entering such a digester, over a period of 20 days and with a temperature of 25-30°C (77-86°F), a very high percentage of the mass is transferred into methane; a small proportion is also passed on to other ponds, some as living cells (bacteria or algae).

As methane forms, the oxygen demand of the effluent falls; about a cubic meter of methane generated removes about 2.89kg of solids, reducing biological oxygen demand (B.O.D.) to that extent.

In the digester, 90-94% of worm eggs are destroyed, as are many harmful bacteria.

Useful energy is generated and can be used at that location to run a motor for electricity or to compress gas for cooking or machinery (or both, as power demands vary).

This motor both supplies the heat for the digester process and also compresses the gas for digester agitation, and for energy supply.

What happens in the digester? The marsh gas produced, [hydrogen sulfide (H2S)] combines with any soluble forms of heavy metals to produce sulfides, which are insoluble in water above pH 7. A little lime can also achieve or assist this result.

Hussainey found that the following result occurred in anaerobic ponds (see original metal content, Table 7.3):

Copper is removed 97%, of which 78% was removed an aerobically.
Cadmium is removed 70%, all an-aerobically.
Zinc is removed 97%, 83% removed an-aerobically.
Nickel is removed 65%, 47% an-aerobically.
Lead is removed 95%, 90% an-aerobically.
Chromium is removed 87%, 47% an-aerobically.
Iron is removed 8S%, 47% an-aerobically. (Up to 92% of iron was removed by the facultative pond process, but some iron was partly dissolved in the aerobic pond again, to give the 85% quoted.)


The results are that solids, metals, and disease organisms are very greatly reduced by the first (anaerobic) treatment of sewage.

What, in fact, happens to the sludge? It becomes methane. In an anaerobic shallow pond, or a deeper agitated pond, the more sludge, the more active the pond.

Thus, a self-regulated equilibrium condition soon establishes where input balances gas output.

If we remove the sludge, the process slows down or stops. This is a clear case of leaving well alone, of active sludge becoming its own solution; rather than being a problem, it generates a resource (methane)Insert Here

In the anaerobic pond, there are few algae, but there are some specialized sulfur-loving bacteria of the genera Thiosporallum, Chromatium, and Rhoda­pseudomonas.

These (in open ponds) may appear pink and give this color to the ponds. They use hydrogen sulfide as a hydrogen source for carbon assimilation; their by-product is therefore elemental sulfur (S), which binds to the metals present.

About 1.8-2.0 mg/1 of heavy metals are precipitated as sulfides at 1.0 mg/1 of elemental sulfur. The bacteria help in this process.

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Passing now to the facultative ponds, we see both the life forms and the biochemical processes change. Algae blooms here; four almost universal sewage lagoon algae are forms of Euglena, Chlamydomonas, Chlorella, and Scenedesmus.

The total algal and bacterial floras (of many species) are called PHYTOPLANKTON (plant plankton).

Bacteria are also phytoplankton, the bacteria benefiting from the oxygen produced by the algae. Typical bacteria in the open ponds are Cyclotella, Pinnularia, Hypnodinium and Rhodomonas.

The sulfur-loving bacteria may linger on in the sludge base or facultative ponds, but are absent or rare in aerobic ponds. The algae fix carbon, releasing oxygen to the bacteria.

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With such a rich algal food available, ZOO­PLANKTON now thrives; most are rotifers (Brachionus, Trichocerca, Haxarthra, Filinia); cladocerans (Daphnia, Moina, Chydorus, Pleuroxus); copepods (Mesocyclops); and ostracods (Candanocypris, Cypridopsis).

Among these are protozoan flagellates, ciliates, and some nematodes. On this rich fauna, waterfowl and fish can flourish.

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Some of the remaining metals are gathered by the zooplankton. In mg/1 (dry weight) they contain 1,200 of iron, 152 of zinc, 37 parts of copper, 28 of chromium, 12.2 of nickel, 10.3 of lead, 1.7 of cadmium-almost a mine in themselves.

Harvested, both zooplankton and algae can be added to foodstuffs for poultry.

Pumped into forests or fields, they provide manures and trace elements for growth.

In rich algal growth, blooms of such forms as Daphnia can be as dense as 100 mg/1.

These zooplankton masses are self-controlled by eating out their algal foods and can in their turn be eaten by fish in subsequent pond systems.

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Of the pH, which varies both long-term and in 24-hour cycles, it too increases from stage to stage: anaerobic pH 6.2-7.8; facultative pH 7.5-8.2; and aerobic pH 7.5-8.5.

In clogged algal waters at night, it may climb higher.

At the aerobic stage, the B.O.D. is only 3-57 mg/1, due mainly to nitrogenous compounds, the suspended solids 32-50 mg/1 (now mainly algae and zooplankton).

About 80% of these have been removed and incorporated into life forms, and the metal levels are now down to World Health Organization standards.

The water can be used for irrigation, or filtered via rush beds to streams.

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Seasonal changes are noticeable. In winter, more hydrogen sulfide is given off by anaerobic ponds (8-15 mg/1 compared with summer’s 2-5 mg/1), and winds may contribute more to oxygen levels in open ponds than do algae; in winter too, more ammonia (NH3) is released to the atmosphere.

Summer sees residues oxidized to nitrates. The oxygen being provided more by algae than by wind, less hydrogen sulfide is given off and there are greater ranges of temperature. In winter (10-l5°C), decomposition slows and sludge levels build up, only to be more actively converted in the summer warmth of 18-22°C (64-72°F), B.O.D. is 495 kg/ha/day in winter, 1034 kg/ha/day in summer (at optimum pond conditions), showing that activity almost doubles as temperature increases.

Consequently, almost twice as much gas as methane is given off in summer (or in heated digesters).

In winter, the cooling water of methane powered engines can provide the essential heat to digesters via a closed loop pipe.

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In all, this simple lagoon series produces a very beneficial effluent from heavily-polluted influent. However, there are even more sophisticated biological treatments omitted-those affected by the higher plants.

As outlined below, some genera of rushes, sedges, and floating plants can greatly assist with removal of heavy metals and human pathogens, but perhaps more importantly, some plants can also break down halogenated (chlorine, bromine) hydrocarbons synthesized as herbicides and pesticides.

Israel (New Scientist, 22 Feb ’79) leads sewage waters to long canalized ponds, agitated by slowly-revolving paddle wheel aerators. Ponds are 0.5 m or less deep.

Under bright sunlight (or under glasshouse covers) dense algal mats form, and these are broken up by the addition of aluminum sulfate (a pollutant!), skimmed off, drained, centrifuged, steam-dried, and fed to either carp or chickens (although I imagine that carp could self-feed on aquatic algae).

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Algal protein replaces 50% of soya bean protein in feed rations to poultry. Total treatment by these methods takes about 4 days.

The water is alkaline and somewhat anaerobic, needing more agitation in winter or on cool days.

Holland runs sewage to similar canals, and reaps reeds or plants as green crop or for craft supplies.

It has been found (Ecos 44, winter ’85) that the artificial aeration of facultative ponds is most efficient if run at intervals of two hours in six (30% of the time).

The facultative bacteria follow two digestive modes and operate best if a rush of air is supplied after a four-hour anaerobic period, excreting carbon dioxide and thus reducing the bulk of sludge.

There are corresponding reductions in energy costs for aeration. Nitrogen was reduced from 20 mg/1 to less than 5 mg/1, phosphorus from 8.5 mg/1 to less than 1 mg/1 when ferric chloride was supplied. The process has been dubbed A.A.A. (alternating aerobic and anaerobic) digestion.

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Thus, agitation of anaerobic systems by bubbling with compressed methane, and A.A.A. of facultative ponds can be used to obtain useful yields of methane and high-protein algae from sewage.

As for the aerobic ponds, such higher plants as water hyacinth removed residual metals, surplus nutrients and the coli group of bacilli (New Scientist, 4 Oct ’79, p. 29).

Microwave radiation can also be effective at breaking up algal mats and sterilizing algal products, eliminating toxic aluminum salts.

As for temperatures, solar ponds used in conjunction with compact anaerobic ponds can supply the low grade heat necessary for efficient sludge digestion and methane will drive any motors needed for both aeration and the gas compression used for the agitation of sludge.

The whole processing system can be made very compact and at the aerobic pond level, throughflow can be led to firewood or fuel forest systems, to irrigated grasslands (as at Werribee), or via trickle irrigation to crops in arid areas.

Final treatment, now in use in Holland and recommended by scientists at the Max Planck Institute in Switzerland, can be released via a sinuous sealed canal of a variety of rushes and floating water plants.

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Waters polluted with metals, biocides, or sewage can be cleaned by travelling through reed beds of Scirpus, Typha and Juncus; (Figure 7.34) or by harvesting off floating plants such as water hyacinth.

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The rushes and sedges can be mown and removed periodically for mulch or cellulose.

For untreated sewage, a holding time of 10-12 days is necessary or travels through a series of maze-like gravel-filter canals with floating weeds and sedges.

For swimming pools and less polluted systems, a pumped “cycle” of water through ferns, rushes, and watercress suffices to remove urine and leaves.

Such pools need a 23-30 cm (9-12 inch) coarse river gravel base, with intake pipes below, and a skimming notch for leaves.

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Species recommended are:

Phragmites communis and spp., Typha spp.: Flocculate colloids, dry out sludge’s, eliminate pathogens.

Schoenoplectus spp.: Takes up copper, cobalt, nickel and manganese; exudes mould antibiotics.

Scirpus spp.: Breaks down phenols, including toxic pentachlorophenol.

Low to zero populations of E. coli, coli form bacteria, Salmonella, and Enterococci are found after water is treated via the above species. Virus and worm eggs are also eliminated.

Also active in pathogen removal are (although these species must be tested and selected for specific problems): Alisma plantago-aquatica, Mentha aquatica, Juncus effusus, Schoenoplectus lacustris, Spartina spp., Iris pseudocorus.

For chlorinated hydrocarbons; use rush types with large pith cells (Aerenchyma), e.g. Juncus spp., especially Juncus effuses, Schoenoplectus spp.

Cyanide compounds, thiocyanates and phenols were treated in fairly short flow times (7+ hours) with Juncus.

Systems must be carefully tended and monitored in field conditions. Water can flow through a gravel base planted to purifying species or for longer rest times, passed through lagoons and ditches.

Domestically, a comfrey bed is one way to absorb the fecal products of animals, where wash-water from yards or pens is available. Comfrey can stand heavy inputs of raw feces in solution, and the crop may then be used for fodder or trenched for “instant compost” under other species of plants such as potatoes.

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Flow through systems for methane production take little plant nutrient from fecal matter, and comfrey or algae ponds deal with the residues, while producing useful by-products for compost and stock feed.

The water from sewage lagoons has been safely used to rear beef cattle at Werribee for 35 years and at Hegerstown (Maryland, USA), sewage waters supplied to selected coppiced poplar plots can produce (as wood chips) some 60% of town energy use.

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Obviously, water saved from reducing the extent of urban lawn systems can supply the remaining deficit plus food crop for any town.

As waters pass through towns, it may gain from 300-400ppm in salinity – a grave factor in usage in any dryland area (New Scientist, 13 Oct. 77).

Saline waters can cause problems in irrigated systems, but algae and plant production and removal will reduce this surplus salinity.

Discharge of sewage to subsoil’s does not remove nitrogen compounds from sewage or farm run-off.

Again, it is necessary to use productive pond production of algae to reduce nitrates to safe levels for discharge to soils or we risk pollution of wells and bores, as has occurred in Israel and the USA.

As with garbage, separation of sewage into solids and liquids at the domestic level has productive advantages; 2% urea sprayed on the foliage of rice plants in paddy has increased grain protein yields to 40% (11% protein by weight; New Scientist, 1 Sept.77).

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Such separation can also be used to recover alcohol and chemicals from urine wastes. Urine diluted with water to a 5% solution controls moulds on cucurbits and aids garden growth or compost activity generally.

In summary, it has long been apparent that modestly-designed sewage treatment systems based on sealed (not  leaky) lagoons and their associated biological systems not only function to recycle water efficiently, but to create a variety of yields from the ‘wastes‘ of society.

There are simply no modern excuses for continuing with the dangerous disposal of such wastes to seas and subsoil’s, where they inevitably turn up as pollutants in wells, streams and on beaches or add considerably to the greenhouse effect of atmospheric carbon dioxide.

It is possible to design small and large systems of water treatment systems which are both biologically safe and productive. (Figure 7.35)

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Creative Disposal of Septic Tank Effluent

There are two basic productive disposal systems for septic tank effluent:

Underground and surface leach fields around which trees are grown.

Biogas conversion, followed by a pond growing aquatic crop for biogas feed stock, then a leach field. (See Figure 7.36)

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A leach field is a trench or open graveled soakage pit through which sewage wastes from a septic tank flows.

In clays and day-loams, tank water from a family home will stimulate fruit tree growth (without other irrigation) for 20 meters or more. The system follows normal procedures in that a long trench with a 1:12 ratio base slope is dug away from the septic tank outlet pipe.

Topsoil is put to one side, and the trench is fitted with an 18cm or larger half-pipe as per Figure 7.37. Coarse gravels or stones are placed in the trench and over all this a strip of plastic or tarpaper is placed.

The trench is then back-filled and trees planted 1-2m off both sides as 2-6m spacing. All fruit and nut trees benefit.

Square or round pits about 25m square can be dug out and filled with graded stone (coarse 6mm at base to 2cm at top). Over this, a layer of cardboard and a thick layer of straw are spread and the latter sown with oats or green crop.

Around the pit, trees can be planted.

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For biogas applications, septic tank effluent, weeds and manures are loaded into a tank 2 to 1.5m deep and 3-4m in diameter. A loading chute for weeds and wastes about 20cm and 30cm slants to the base.

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Septic tank effluent also enters at the base. Overflow goes to a pond with baffles and Pista, watercress or any rampant soft water weed is grown there.

These are returned to the tank every week. A perforated pipe at the tank base is worked by a small gas compressor to “bubble” gas back into the tank for 1-3 hours daily on a timer.

This breaks up the scum on top of the ferment. Gas caught in an inverted tank is fed to the house cooking range, lights, and refrigerators.

Surplus from the pond is fed to a leach field. (Figure 7.39)

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