THA: 75/008/79/WP/8 APPLICATION OF LOW-COST FILTRATION SYSTEMS TO FRESHWATER PRAWN CULTURE IN THAILAND

by

George S. Cansdale
S.W.S. Ltd., England
(on assignment to FAO)

(FAO/UNDP/THA/75/008)

Bangpakong, Chacheongsao
Thailand
1979

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1. INTRODUCTION

This working paper represents those parts of my consultants' report to FAO, for the period 20 August – 24 September 1979, which relate to the Programme for the Expansion of Freshwater Prawn Farming in Thailand (Project THA/75/008). Full details of all the techniques and units utilised can be found in an earlier report published by the South China Sea Fisheries Programme^a.

The primary reason for visiting Bangkok was to help in solving problems of unsatisfactory water in Macrobrachium hatchery systems. Methods of abstracting pre-filtered sea water from the natural bed were taught and had already been applied before the visit ended. The technique of using both Village Units, for primary filtration, and Mini-Units, for recirculation, in wholly artificial beds, was shown in Chacheongsao: in this case also a permanent source was created and this had been supplying clean water for some four days before the mission ended.

Abstraction from insitu beds is clearly impossible on the vast Central Plain, where the canal's base usually consists of almost bottomless mud. For the numerous isolated communities who still take much of their household water from these canals it seems that the mini-Unit methods could bring much improved water at minimal cost. Throughout this region there are numerous fish ponds. While it can hardly be economic to filter the whole supply it seems that the general method described below under “Chacheongsao Fisheries Station” could well be used for that part of the supply needed for the actual hatchery and the early fry stages, as well as for any sensitive species that might be kept.

^a Cansdale, G.S., 1979. Low-Cost Water Filtration System. Publication SCS/79/WP/80 dated February 1979. South China Sea Fisheries Development and Coordinating Programme, P.O. Box 1184, MCC, Makati, Metro Manila, Philippines.

It was clear that many beaches to the east of Bangkok are ideal for providing by sub-sand abstraction the marine part of the water supplies for Macrobrachium culture, removing any need for settlement or filtration. It is understood that the same is true of beaches on the Isthmus of Kra. It is likely that this method will be adopted fairly widely, in places with the use of homemade hardware, resulting in much saving of labour and improved results. The application to fresh water is equally valuable since in many hatcheries this can be taken only from the canals and is high in silt: the provision of sediment-free water is directly helpful to the stock and also allows fuller use of water.

Thanks to the initiative of Michael New, many contacts were made with a wide range of persons interested in water, whether for human use or in fish-farming, and discussions were held with all those listed in the diary (Appendix IV). It was unfortunate that the nearest practical sites for demonstrating these techniques were at Chacheongsao and Bar Lamung, but a total of some 25 people were present at the former or both sites on the 21st September 1979.

General instructions for installing the Village Unit are given in Appendix I. The results of work carried out for the freshwater prawn project in Thailand are given below.

2. CHACHEONGSAO FISHERIES STATION

The Fisheries Station uses brackish water which can be drawn direct from the canal at only two periods of the year. During the rainy seasons the estuary supplying the canal has fresh water, so that salt water needed for mixing is drawn from storage. In the dry seasons the BANGPAGONG estuary is salt and fresh water is drawn from the reservoir. Current total needs are for c. 200 m³/d or 8 m³(1800 g)/hour. Storage capacity is adequate. The station has a number of large earth ponds but an almost unlimited depth of mud makes it impossible to construct an artificial bed in any of these. There are no natural sand beds but unlimited suitable sand/gravel is obtainable nearby.

An artificial bed with an approximate surface of 5 m × 5 m was prepared in one of the large nursery tanks with a cross section as in Appendix II, Fig. 3. The central core was of 30–50 mm stones. The Unit was pre-packed with c. 2–5 mm gravel; the remainder was sand/gravel as supplied, of part river and part beach origin, and was a mixture ranging from below 0.5 to 5.0 mm. The tank was filled with partly settled raw water. After fifteen minutes the clay fraction in the bed was largely evacuated and water was clean and physically acceptable for use, though still with a faint opalescence caused by a small clay particle. The water was then fed to the main system and the quality steadily improved over a further 2 hours' pumping.

A Mini-Unit was established in an existing sand bed within one of the smaller hatchery tanks where the recirculation water was opaque with both mineral particles and organic debris. The Mini-Unit was developed and run by gravity only, in the absence of a suitable hand or small electric pump, and it gave excellent quality. A reducing head of water made volume measurements difficult but c 60 cm head the flow through a 20 mm pipe was c 0.5 m³h. This flow will be much increased by full development using a suitable small pump.

The Mini-Filter was demonstrated in the sand already part filling the centre of a concrete tank and serving as a sand filter, through which the water passed horizontally. This seems potentially dangerous since the water probably travels through only part of the cross section, keeping this aerobic, while the remainder may become anaerobic. This may not cause any trouble but it is better to organise circuits so that the whole bed is fully used. A more positive method would be to use a series of Mini-Units at about 1 m centres in a bed of sand/gravel 50 cm deep, the space above being filled with water. The levels overall do not allow gravity working, but this layout would probably make it possible to use air lifts for each individual Mini-Unit. Formulae are available to calculate flow in an air lift system but it is generally best to work out details by trial and error.

The Mini-Unit is proving effective when used in a container in aquarium circuits and in open small ornamental fish ponds, and it is suggested that this is tried here, with careful monitoring (Appendix III, Fig. 5). This pack can safely stand actually in the tank, for the rate of downward flow to the filter surface will not draw down the larvae. Cleaning should be infrequent, especially if nylon wool etc. is placed on the filter surface to act as a strainer, but it is easily achieved by lifting out one pack after putting in another a few days earlier to build up efficiency; or preferably, having sufficient Mini-Units working to allow one to be taken out at any time for maintenance.

There was general agreement that even after the short period of development the water from the larger filter was physically more than adequate for hatchery use, but for recirculation systems the emphasis was placed on chemical parameters, in particular ammoniacal nitrogen. Conditions in Thailand are markedly different from those in temperate zones and monitoring of the relevant parameters is necessary, but we are confident that if flow rates and bed size conform to those suggested, the biological filter zone will be more than sufficient to process the ammonia and other break-down products in normal systems, even when more heavily stocked with larvae than at present. These rates should therefore be taken as a starting point and in many cases, after test-running, it should be possible to increase the flow with safety. In English outdoor conditions it is found that bacterial removal and reduction of BOD and ammoniacal nitrogen run almost parallel and that the percentage removal remains more or less constant over a range of raw water values, the former remaining steady at about 97% and ammoniacal nitrogen at about 90%. In Thai systems the temperature will be around 27°C, at which the biological activity is much greater.

3. SITES AT BAN LAMUNG AND CAPTAIN HUGHES BAY

3.1 Seawater Intake

All the beaches examined had sand of above 3 m in depth in the upper section, with ideal particle size distribution for forming beach wells. At Captain Hughes Bay, the colour of fine material evacuated during development suggested a minor degree of pollution: this is only tentative opinion and it is not important, for the site was pumping particle-free water at full volume in under 10 minutes. If a hatchery is established in this bay an unlimited flow of high quality pre-filtered water would be easily obtainable.

Ban Lamung is the site of a private hatchery and the upper beach is perhaps the most favourable on which the SWS system has ever been tested, for it was pumping at full volume and high quality in about 5 minutes. The pump outlet was then attached to the existing rising main and, for the time available, clean water was delivered to the working systems nearly 400 m distance away, through a 50 mm pipeline. The flow was assessed at c. 10 m³/h, causing a friction head of at least 16 m, plus 2 or 3 m static head. Since the pump had a flow of c. 30 m³/h through a short delivery pipe, the rising main is the limiting factor. If a larger pump was used to increase the flow to 13 m³/h the head would then increase to over 30 m and the connection and lower part of the pipeline itself would be in danger of damage. A new 75 mm line would carry at least 25 m³/h without excessive head but the extra cost would not be justified. It is simpler and much cheaper to use the existing system and pipeline for a longer period daily. Establishment takes such a short time that it is probably best to insert the well daily and remove both pump and line when pumping is complete.

The first work at Ban Lamung was done when most of the beach was covered by the tide. A subsequent visit at low tide allowed more detailed exploration. While the more steeply shelving upper beach has a great depth of mixed sand, the lower and more level zone, most of which seems to be exposed only at very low tides, has a different texture, being of fine sand and silt with numerous broken shells. If it became necessary to pump continuously this zone could be prepared as a source, incorporating coarse sand from (the upper beach) perhaps 30 metres away. As demonstrated on site, the SWS Unit is more suitable than a screen well in such terrain, since it is easy to gravel-pack, while the much greater perimeter makes it easier to pump at above the critical flow needed to evacuate fine particles and so develop the source. In addition, it is much more stable near the surface than a screen well. The area around such a permanent intake (about 3 m radius) should be worked over very thoroughly by jetting water through it via a steel probe to a depth of c2.5 m, to remove as much as possible of the fine material before a beach well is installed.

Since the upper beach can be used for several hours over each high water, and, apart from spring tides, almost continuously, it will be simpler to pump from the locally made PVC point installed during my visit and left in position as low down the beach as possible in order to give maximum hours of pumping while keeping the well in the coarser medium. This will tend to block easily but it can be taken up, cleaned and replaced in a very short time. Alternatively, it may be preferable to install daily at a suitable point the stainless steel screen well which was provided for Ban Lamung.

3.2 Freshwater Intake

The site at Ban Lamung has fresh water which currently has a sufficiently high iron content to stain all tanks, so that advice was asked about possible treatment. The well has been pumped for only a few months on a routine of 3 or 4 hours daily: the iron precipitates slowly and most comes out of solution in the working tanks. Iron in water can range from solid particles to chemical forms which are hard to precipitate, and while aeration usually converts the more soluble ferrous into the less soluble ferric compounds, reaction may be delayed. The following suggestions are made for reducing this nuisance.

1. The opinion was expressed by local staff that the iron being sedimented is now rather less and this was again confirmed at the end of my visit. It may be that instead of being in the water itself iron is being leached from the bed: if this is so the amount is finite and in thime will cease to be noticeable. It is not uncommon for a bed to be cleaned just by pumping, and to expedite this the pump should be run as continuously as recharge rate allows for several weeks, any surplus being put into the ground at a distance from the well, while monitoring weekly for iron. An iron testing kit is available.

2. Construct a simple cascading column from an old oil drum or similar, putting in two perforated supports to hold coke, broken breeze blocks etc. on the top layer and ditto with some finer gravel (1/2" to 2") on the lower. The first serves to give thorough aeration and a long contact time with the air. The lower layer takes out the coarse floc. This is simply made from scrap and quickly tested. If it works, several can be made and used alternately to allow cleaning out etc. (A more elaborate and expensive version of this device can be seen at NIFI.)

3. In some cases iron settles on the pipe wall to form a coating of 'iron bacteria' which are oxygen-demanding, and it may be helpful to run an intermittent pumping regime something as follows:

   1. Switch off pump, making sure that the non-return valve does not allow the pipe to empty.

   2. After an interval switch on again and pump to waste.

   3. Allow the water to go into system.

The time needed for (a) (when the iron bacteria run out of oxygen and become detached from the wall) and (b) (when the dirty water is run to waste) can be found only by experiment, beginning with a twice-daily stop for about 15 minutes and working each way from this.

If none of the above is helpful, a series of settlement tanks plus added chlorine or air may be needed: special iron filters can be bought but these are more expensive and need careful maintenance. We can only express the hope that the suggestion in (i) above is confirmed.

APPENDIX I
INSTALLATION AND OPERATION OF UNITS

Fig.1. Diagram of SWS Box Unit and Water Flow

INTRODUCTION

The SWS Unit is easy to install and run. Once the principle is understood it can be used successfully in a range of situations and for many purposes, but it is not a magic recipe for instant free clean water. The advice in this leaflet must be studied carefully, giving special attention to:

Choice of site, pump and pipe line.
Correct installation and thorough development.

These instructions should allow most sites to be worked but there is great advantage in having a small trained mobile team to establish units and teach other groups. This basic training can be given by SWS in Britain or overseas. It is possible for an experienced person to work alone but better to have a team of two or three, of which one has some experience of machines. Technical advice should be asked about difficult sites and also about projects where large volumes are needed.

GENERAL PRINCIPLES

The SWS Unit is not itself a filter but a device for making the sea or river bed serve as a natural sand filter. Dirt is held on the bed surface as water is drawn to the open bottom of the unit through an area of up to 10 m diameter. Water moving over the bed cleans it, helped by fish etc. that poke about and disturb the sediment. In the sea the action of waves and tides is effective. The bed itself becomes a biological filter that destroys bacteria and reduces the levels of ammonia, iron, BOD etc.

SITE REQUIREMENTS

A permeable bed is needed at least 60 cm deep, but preferably not less than 1 m, and under a minimum of 30 cm water. Different types of site, including those to which sand must be added, are described on pp. 3 & 4. Beds of a wide range may be used - sand, gravel, broken coral, shell etc. The bulk of the grains should be between 0.5 and 5.0 mm (0.02 and 0.2 in) but a great advantage of this system is that during development, described below, excess fine sand is pumped out, leaving the larger grains in and around the unit so a precise sand specification is not needed. Uniformly fine sand, especially of wind-blown origin, is unsuitable on its own, but it can be graded up by adding course sand or gravel under and around the unit. If most grains are above c. 2 mm (0.1 in) it helps to add fine sand on the surface around the unit during development. “Fine sand” is material up to c. 1 mm and “course sand” from c. 2 to 5 mm, but these are not used as technical terms. A few stones up to c. 50 mm (2 in) do little harm but larger stones reduce the bed's efficiency and should be cleared around the unit.

INSTALLATION

Dig a hole with centre deep enough to take the unit, open end down, so that its top is up to 15 cm (6 in) buried when stabilised. (See Fig. 7, App. 10) Sand is filled in around the unit and more piled over to allow for settlement. If a hollow forms, more sand should be drawn over it.

A special tool is supplied with each unit, or set of units, for digging this hole. A 2 m pole is fastened to it and it is used like a rake.

In cold climates, most work can be done in waders but sometimes a skindiver is needed, especially in the sea. Men can work comfortably in warm water but special precautions must be taken in bilharzia areas.

DEVELOPMENT

Thorough development is the key to success and this section is most important.

When the unit is buried and the suction line has filled with water, this is then connected to the pump intake. Tight joints with washers are essential, for the smallest air leak delays priming and lowers efficiency. Under-water leaks may admit raw water though if these are very small, they soon block. Develop with a temporary pump close to the water or work from a boat, especially if the permanent pump is to be submersible. It may help to stand on the unit, rocking it gently to settle it until the pump primes fully: the speed should then be reduced until it runs steadily. At first the water if full of silt and organic matter as it cleans the bed. Varying with the site, this water clears in anything from a few to many minutes. The pump is then stopped and restarted; after a very short interval the water becomes dirty but soon clears, when the pump is again stopped and restarted. Releasing the partial vacuum disturbs the sand in and around the unit, allowing more fine material to be sucked out and gradually pushing back the perimeter of clean coarse sand, so improving flow to the unit, which is one of the basic reasons for development. This process continues until water no longer becomes dirty after restarting and the pump is working to full capacity. The type of bed and the pump size determine whether this takes an hour or perhaps a whole day.

Where the bed has much black organic matter, development is best spread over several days to allow this to decay aerobically, after which it is easily sucked out.

The water should now be crystal clear, free of all suspended matter and organisms down to about 1 micron and sometimes below that. It is suitable for most purposes, including for village supplies where the water had previously been used raw. Where sea water is wanted for research or fresh water for town supply, it should be pumped to waste several hours daily for atleast a week while the biological filter develops in the bed, the time needed varying with temperature and other factors. Where quality is critical, this should be monitored.

Where adverse site conditions impede progress the following procedures may be tried:

1. Using a garden fork or similar tool, dig the bed well around the unit, letting the stream carry away much of the silt.

2. Instead of just stopping the pump, release the suction completely, letting the water flow back to the unit, before re-starting.

3. Interchange intake and take-off hoses and pump back for several minutes. Somebody should stand on the unit until normal pumping re-starts.

Small amounts of sand (perhaps 10 ppm) may be drawn through for some days, especially when pumping is periodic, but this is sterile and settles at once in a reservoir or small baffle chamber.

If pump and pipe line are not limiting factors the rate of flow is largely determined by the site and large volume is often possible. For drinking water it is better to under-pump and a steady flow of not above 20 c.m./hour (4,500 gal.) per unit is suggested. Although the system is designed for continuous working in both sea and river, it is equally efficient when pumped periodically, preferably daily, but the system has been shown to be clean and effective in a British river after being left for 3 months. When the pump is started again it is probably advisable to pump some water to waste; this need be for only a few minutes after a day's gap but perhaps for up to an hour after a week. Only local experience will show what is needed and in fact whether this is even necessary.

SITES WITH AMPLE SAND

1. Rivers and Streams

Conditions vary with climate, shape of ground, geology etc. so that the site must be chosen carefully. It must be stable and free from scour, which prevents the formation of a good filter bed and may even wash out the unit. The inside of a bend is often suitable. There should always be at least 30 cm water over it. In rivers which fluctuate widely the only permanent source is one covered at low water but as the water rises it may be necessary to take up the unit and replace it near the edge.

In rivers which stop flowing in the dry season and then disappear, the unit can sometimes be buried below the water table. Drawing the water by pumping instead of from an open hole has two big advantages: since the water is not exposed to sun and air there is less loss by evaporation and the source does not become polluted.

2. Water Catchment Dams

A bank of sand often forms where the stream enters and this should take a unit satisfactorily, but if the dry season level drops too far the unit must be taken up and reinstalled suitably. Wind and water movement should keep the bed surface clean but if it becomes blocked this is dealt with as advised under SITE CLEANING AND MAINTENANCE.

3. Sea Shore

The unit should have at least 30 cm water over it at spring low tide. In very permeable sand on a level beach a unit can sometimes be installed above LTM; it must be as deep as possible but unless water has free access to it at all times, flow may be limited at low tide. Salinity must be monitored during site selection if water is wanted for Aquaria, Marine Laboratories etc., for areas may be diluted by freshwater run-off or beach springs.

In contrast to rivers and dams, sea movements are daily and predictable, and a known factor in the suction head. Tidal pattern may vary widely: rise and fall may be from under 2 m to 15 m (6 to 50 ft) and the tide may recede anything from a few metres to over 500 metres.

SITES WITH LITTLE OR NO SAND

Soft mud areas cannot be used, but where the bed is of rock or clay it may be possible to apply the following general method. Excavate or blast a hole. The shape need not be regular but for top quality water the surface area should be about 2 s.m. per c.m./hour, with depth of at least 60 cm. Usually a much lower ratio is possible. The bottom should be filled with coarse gravel or small stones to a depth of 20 cm to prevent the unit from sealing itself on the bottom and also to provide good lateral access for water. The unit is then placed in position and water allowed to enter, after which the sand is poured in until it forms a hump over the hole. If 2 units are needed they should be well spaced in a long trench. This variation clearly needs more work than a standard site but it can be applied to such as the following:

1. River

Sometimes it is possible to work on the actual river bottom, perhaps after putting in a coffer dam. Or a site may be prepared alongside the stream, to make an artificial 'bay' when complete. It is essential to avoid reaches with massive scour in the rainy season.

2. Storage Dams

These are sometimes lined to stop seepage and care must be taken not to cut through this lining. Two prepared holes may be needed, to draw water at various levels.

3. Foreshore

The inter-tidal zone is sometimes too compact for direct use but conditions vary so widely that only general suggestions can be made, and technical advice is needed, especially where large volumes are required.

On most shores the problem is to excavate holes near enough to LTM to allow continuous pumping. Where there is much beach run-off a long trench parallel to the shore line, plus the water in the trench, may allow at least an hour's pumping while the tide is off it. Perhaps a narrow trench dug seawards can give water access to the trench even at low tide.

Where only moderate volumes are needed the hole can be dimensioned to hold a reserve of say, two hours pumping over low tide. The simplest and cheapest solution is an excavation well up the beach which is pumped for a few hours while actually covered by the tide. This reduces suction head and run to a minimum. The formation of a hollow must be avoided where sea wrack etc. can be trapped and buried. Such work is seldom easy but it is well worth considering, especially on a shore where the tide recedes so far that LTM is beyond practical reach.

PIPE SIZE AND SUCTION HEAD

Resistance (friction head) rises rapidly with both rate of flow and reduction in pipe size, as this table shows clearly. Bends, valves, etc. add to this resistance (for details consult a textbook on Hydraulic Engineering). 40 mm (1-½ in) is unsuitable for over 5 c.m./hour (1,000 gal), while 50 mm (2 in) pipe can handle 18 c.m./hour (4,000 gal) if the pump is close to the water and not more than about 3 m (10 ft) above it. For any flow above 20 c.m. (4,500 gal) at least 75 mm (3 in) pipe is essential.

Failure to understand the importance of total suction head (i.e. friction head plus static head, or height of pump above water level) can be both expensive and disastrous. A pump can suck only from a limited depth; a maximum of c. 7 m (23 ft.), and with most pumps the flow has already dropped severely before this is reached. This is reduced by 1 m for every 1000 m altitude (1 ft for 1000 ft). The suction line should therefore be dimensioned to have no noticeable resistance at maximum flow.

This is not so critical on the delivery side but the figures in this table apply equally and the higher cost of a larger pipe line is recovered in economy on pump size and power used. As diameter increases the carrying capacity of a pipe rises much more steeply than cost.

PIPE LINE

Flexible armoured hose is needed from the unit at least as far as the highest water level. It can be expected to last for at least 10 years. Semirigid PVC type can be used from the river edge and also for the delivery line. For river sites, local conditions will decide whether the line should be protected from vandalism, heating up etc. by burying or in some other way. In the sea it is better to bury the line up to 50 cm deep if the terrain allows, or else secure it to rocks and so reduce damage.

PUMPS

Land-based pumps must be self-priming, i.e. be able to suck air from the suction line and pull water until there is a continuous column from unit to pump. The method of priming varies and instructions in the pump manual must be studied. However large the complex of units, each must be developed individually, using a petrol-driven pump of 3 to 4 h.p. able to pass 2.5 mm (0.1 in) solids. Machine pumps should not be considered for village supplies unless the power to run them, whether electricity or petrol, can be guaranteed. Once the system has been established a Patay hand-pump is recommended for regular use; this easily yields 4 c.m./hour (1,000 gal), it is strong and reliable, and its only running cost is a new diaphragm (under ₤2) every 400 hours running. Wind-powered pumps are not suitable for development but could perhaps be used for filling reservoirs etc. if a hand-pump is always kept in reserve. A simple reservoir holding 2 or 3 days supply makes the whole system most useful.

In the sea a shore-based pump should be used when the static head allows but it must be safely housed above HTM. In a marginal site a sunk pump chamber, thoroughly watertight, may reduce the static head by up to 1.5 m. Tidal pattern and beach shape sometimes make a submersible pump essential; the normal grille is replaced by a special fitting connected by flexible hose to the unit(s). The pump should be mounted firmly on a pile or similar support at a point just exposed at spring low tide for easy maintenance. Submersible pumps are powered only by electricity through heavy armoured cable. It may be undesirable to bring this down a beach to which the public has access but some types can be well protected by running the cable down the rising main, emerging by a special duct at the pump. A small submersible pump is now available that can be housed directly on top of the unit.

SITE CLEANING AND MAINTENANCE

In marine sites tidal and wave movements keep the surface clear and this is also true of most river units. If any blocking occurs this will be in the top 1 – 3 cm, usually 1 cm, and this is most likely in the still water of storage dams. If reduced flow, not due to pump or other factors, suggests that there is surface blocking the following progressively vigorous actions may be taken:

1. Stop the pump and rake over an area of about 5 m radius around the unit, working to a depth of about 5 cm. Then pump to waste while redeveloping as long as needed.

2. Skim off about 3 cm sand and replace with new sand.

3. Fork over the area lightly and then back-wash by moving suction hose to pump outlet and drawing water through spare hose. Somebody should stand on the unit until the pipes are changed back and the redevelopment starts.

4. Remove unit and install and develop in nearby site.

3. and 4. will be needed seldom if at all. In a system with several groups of units, provision for back-washing can be made with little modification to the plumbing. If the suction lines are long this can be used to fill them with water and so reduce priming time when restarting.

A change in tidal pattern or badly sited breakwater may remove a metre or more of sand from the beach, though this is unlikely near or below LTM. If the unit becomes exposed it must be installed afresh.

The contours of a river bed may change, making it necessary to find a new site, though intimate knowledge of the river, with a careful preliminary survey, should allow the selection of stable reaches. Where scouring occurs it may be enough to replace the sand and develop again, but the trouble will probably recur and it is usually better to move the unit.

A firmly embedded unit can be quickly freed by changing over suction and delivery hoses at the pump and blowing back, after letting some air into the line to the unit.

APPENDIX II

APPLICATION OF SWS SYSTEM TO FISH PONDS

Since all ponds have impermeable bottoms, whether natural or artificial, beds for housing SWS Units must be introduced. The following procedure is suggested:

1. Medium

Most sand seems to be dug from river beds where it is constantly being deposited. Much contains a high proportion of particles below 1 mm but the quality varies and average particle size is larger when the flow is fast. Sand should preferably have not more than 10% below 1 mm with most between 1 mm and 4 mm. If the sand is fine and dirty, it can be quickly washed in a concrete mixer with water hose running, the best procedure being found by trial.

2. Siting

A corner of the pond is best. Its existing banks from two sides, the others being made by walls of roughly piled rocks or blocks. See Fig. 1 and 2. If the area is close to the overflow to next pond, it has most water movement over the surface. Nearly all pond water is already dirty and it is essential to draw from a pond where the mud is not being stirred up by fish such as large carp, for this can add unnecessarily to the filter load. It is best to use a small pond without any fish or divide off a section of a large pond containing only small or surface fish.

3. Size

An area of not under 3 m × 3 m is suggested, with a final total bed depth of c. 65 cm in centre, i.e. including Unit, and tapering to c. 30 cm at edges. See Fig. 2 and 3.

Good sand is available at very low cost in most parts of the country and the biggest beds practicable should be made. These will allow maximum volume and also last longer between maintenance. Even a bed 5 × 5 m and 75 cm deep would cost under 20,000 Rupiahs.* At 24 m³/h, this is equivalent to a surface flow rate of 16 1/m²/min, which is within the flow for maximum quality slow sand filtration.

* This section of the report applied to conditions in Indonesia.

4. Construction

Although the work can be done while the pond is full, it is easier and quicker when pond has been drained for cleaning. The operation is as follows, all figures being taken as approximate:

1. Clear away mud to leave hard bottom.

2. Complete sides by building rough walls of rocks, old bricks etc. 30 cm high.

3. To allow good access of water and keep the Unit above clay bottom, two different methods are possible and the more convenient should be used:

   1. Put down a circle (1 m dia.) of stones 5 cm diameter and depth of 10 cm.

   2. Put a large rock or block 10 cm. high under each corner of Unit. See Figures 2 and 3.

4. Pour sand to depth of 15 cm and put Unit in position. Press down firmly but do not stand on it. If pond is empty, let the water start filling it.

5. Add gravel/sand to water (to stop forming air pockets) until centre has total depth of 65 cm sloping to 30 cm at edges. If a large stone of 15–20 cm thickness is placed on top of Unit, it indicates depth of sand and holds it steady.

5. Development

Thorough development is essential - it may take a whole day, with many stop/starts to reach maximum volume and top quality. If the sand contains much fine material and progress is slow, it will help to blow back briefly at low speed several times while somebody stands on the Unit to hold it in position. “Briefly” means not more than 2 or 3 minutes after the water has reached the Unit. Several times in this way will be more effective than a longer single period.

As explained elsewhere, in the high ruling temperatures of this area, continuous running is essential to maintain the aerobic conditions required by a biological filter. Once development by petrol-driven pump is complete, this should be replaced by a 2 or 3 h.p. electric pump.

6. Water Quality

The raw water in the Sukabumi and Grobogan fish ponds was more polluted with organic matter than in any previous tests. In both, there seemed to be complete removal of all algae and organic particles, leaving only faint milkiness caused by fine clay particles already discussed. These particles are probably too fine to be taken out by any filter. At the end of development, particles will be removed to c. 2 microns including Lernaea.

After c. 7 days of pumping, the bed will become a biological filter and the following extra results are expected:

1. Reduction of ammonia by c. 80% and of BOD and detergent by c. 50%.

2. Removal of above 75% of all bacteria but this could be as high as 98%.

3. Marked reduction in D.O. level, since the oxygen has been used to oxidise polluting materials. This is simply restored by cascading etc.

Note: It is difficult to give definite figures for quality, because

1. Conditions in these ponds vary widely, especially as regards algae, both free and surface.

2. The temperature is higher than in any other outdoor work we have done. The biological activity is, therefore, high and results may be better than suggested.

7. Maintenance

Since most material removed is probably organic, this should break down leaving few solids, but only experience will show how quickly the volume is reduced by blocking. Simple raking of the surface followed by redevelopment while pumping to waste should not take above 2 hours, and may take much less. It may prove useful to do this on a routine basis.

Conditions are very different from ponds in temperate zones. It is suggested that a system is established at one or two centres and fully observed for at least a month before it is used generally, but it should give much improved water for hatcheries and fry-rearing.

Need to be Patient:

Full preparation of site and very complete development are essential, even if each may take a day or more. A source is being established for use during months or even years and there is no point in trying to do this in the shortest time. Once the system has been formed and development begun, it may be enough to leave a workman in charge, stopping and starting frequently, and gradually bringing the volume up to full flow.

Although it is possible only in hilly country, gravity abstraction from SWS Unit is worth considering for providing clean water to breeding ponds at no cost for power, once the system is fully established. It can run by siphon but it is better to bring a pipe through the pond wall and thus have a permanent head of water. In theory, a total head of 1 m, i.e. from water surface to outflow, can give a volume of c. 30 m³/h from a 50 mm pipe. In such condition, considerably less could be expected but the pipe should be fitted with a control valve and also a U-joint to keep the line full of water.

The screen well, which is ideal for working at depth, is not recommended for artificial beds or shallow formations, for the following reasons:

1. It appears easy to insert but it is equally easy to disturb. In contrast, pumping holds the SWS Unit down.

2. Any erosion or removal of sand can quickly expose the screen to raw water.

3. Most important, when the Unit is installed and covered, all water must travel down through the sand to the depth of the Unit and then up. In fact, most of it travels much farther, from several metres. There is no such minimum imposed by the well screen.

4. The bed can be much more fully developed by the Unit.

Fig. 2. Section (Not to scale)

Fig. 3. Section (Not to scale)

Final Note:

The construction of artificial beds for obtaining improved water from fish ponds is described above, with the possibility of running by gravity, thus cutting out cost of power after original establishment. This method is being used in several countries for supplying water to isolated communities in hilly areas.

The Unit is placed in a sand-filled collection area, either natural or artificial and a pipe is then taken through the containing bank or wall to ensure a permanent head of water. In some cases, only a ½" pipe feeding into simple storage and running to waste is needed. This can provide 24 m³ daily from a sand bed of 2 m², and unless the source is grossly polluted, the water will be close to western potable standards. We believe that this possibility is worth notifying to authorities concerned with water in areas beyond the reach of piped supplies.

APPENDIX III

FILTRATION FOR SALT OR FRESHWATER TANKS

Although the following note is mainly to meet the need for simpler and more effective filtration at the Brackish Water Centre, Jakarta, these two basic methods are effective for salt or freshwater tanks generally. Both are economical in space and material and are simply maintained. They are essentially biological filters and equally efficient, and are being used successfully in large tropical aquaria in U.K. with heavy stocking. The choice of type is by convenience, for one is likely to fit more easily into an existing circuit than the other. Designed for flows of up to c. 1 m³/h they can easily be scaled up by having 2 or more systems in parallel.

External Plastic Container

A dustbin (trash can) of 80–100 1. capacity is ideal, but if an old plastic drum of similar size can be found that is strong enough, this is equally good (Figure 4).

Fig. 4. Drum Unit for Salt or Freshwater Tanks (Not to scale)

Make a hole close to bottom and fit suitable outlet. Unless the container is to rest on a concrete slab, prepare a strong support for when full it may weigh above 200 kg. When in position, fill to within 15 cm of top with 2–5 mm gravel. Cover surface with sheet of glass or nylon wool in such a way that all effluent must flow through it. This sheet removes suspended matter and is changed or cleaned from time to time as it becomes visibly blocked. The mass of gravel becomes a biological filter within 7–10 days in a temperature of 26– 30°C and this should be left undisturbed as long as possible, certainly for at least 6 months. When this must be cleaned, some should be set aside, unwashed, to serve as culture to start biological filter quickly.

This method is most suitable when the existing main tank overflows to filter, storage, etc. Within reason, the larger the container the better, for it allows the water to flow over a larger active surface.

SWS Mini-Unit Housed In Gravel

This involves the use of a small electric pump - c. 200 watt - which may already be in use for circulation. An old container as discussed above, is cut in half and filled with gravel in which the mini-Unit is housed. The grade specified above should be used for the greater part, with a topping, 4 or 5 cm deep, of clean, well-washed sand. See diagram (Figure 5).

Fig. 5. SWS Mini-Unit Housed in Gravel
(Not to scale)

This is placed conveniently in the tank and serves as a self-contained sub-gravel filter. Massive development is not needed but it should be stabilised by pumping to waste over 3 or 4 stop/starts for, say, 10 minutes. This biological filter takes at least a week to become effective and it may help to add a little sand or debris from the tank bottom to seed it.

Standing clear of the tank bottom, it receives little silt: even in heavily loaded and fed tanks, the flow remains stable for 4 and 5 months without attention. The simplest way of cleaning, when reduced flow makes it necessary, is to remove the container, wash the sand/gravel and replace, also keeping a small amount of dirty gravel for seeding. This is suitable for systems housing small crustacean larvae, for it works by suction at a flow rate that does not draw down the larvae to the bottom surface. Accumulated fibrous debris on the tank bottom needs to be removed by syphoning from time to time.

Both of the above filters will process ammonia and other metabolites and reduce the need for water replacement, especially if the systems are only lightly stocked with larvae. It must be noted that this biological activity uses up some of the D.O. and this must be restored by cascading, injection, etc. One simple device for this is illustrated. Particularly with sea water, if there is any risk of a diminished pH, it is useful to use coral breakdown sand as a medium. This is easily obtained from many localities.

Fig. 6. Replacing oxygen by cascading the water.

APPENDIX IV

DIARY OF WORK IN THAILAND