A few people have asked me recently ‘what do I know about septic tanks?’, which made me realise that I know very little. To date my contribution to sustainable wastewater management has been a note on my drawing saying ‘pipe connected to new septic tank designed by contractor’ or even better ‘pipe connected to existing septic tank’.

Realising that I need to be a little more responsible than this, I have been googling all sorts of interesting subjects and this is a summary of what I have come up with.

Most septic tanks in Sabah look something like the above sketch. They are made of either plastic or concrete and usually discharge straight into a river or open drain. The effluent (discharge) pipe takes water from well below the scum layer and well above the sludge layer. Each time a new delivery arrives at the influent (inflow) pipe it will displace an equal volume of effluent.

There are two very important considerations:

1. Time is needed for bacteria to digest the waste. The tank needs to be large enough that fresh influent can sit for a while before being displaced as effluent. Obviously a higher volume of water flushing through the tank will need a larger tank.
2. Sludge will get deeper and eventually need to be removed. This is usually done by a ‘honey dipper’ as the Americans euphemistically call it. In reality it’s a tanker truck with a big dirty vacuum cleaner. In Sabah I have discovered that they don’t have this kind of truck so the sludge has to be removed by hand. Often immigrant labour will be used for this job which makes it more acceptable, apparently. On many of the sites I am working with there is no vehicular access anyway. In which case when the tank fills with sludge the only viable option is to seal it up and build another one.

Above is an illustration of what happens if the sludge is not cleaned out, but instead allowed to reach the outflow level of the effluent pipe. Any new delivery will cause sludge to be displaced as effluent. Clearly this is not good. In some cases the effluent pipe will become blocked and scum followed by sludge will come out through the inspection hatch or back up into the building. Clearly this is not good either. It is usually fairly obvious when this starts to happen because of the smell.

When correctly maintained this septic tank system works well. Plastic tanks are available cheaply off the shelf and concrete ones are easily built insitu. This system is apparently favoured by the Malaysian Ministry of Health.

A better system is to use dual tanks (or one tank with two compartments) which allows more control of the digestion process. The first tank has a mix of undigested and partially digested waste, whereas the second tank contains only partially digested and completely digested waste. Turbulence caused by fresh influent does not stir up the waste in the second tank.

Dual tanks will also give a better chance that wastewater will stay in the system long enough for waste to be fully digested. Dual tanks still need to be de-sludged, although perhaps at reduced frequency in the second tank.

In Sabah it seems acceptable to discharge effluent straight into the environment, even from a single tank system. In Western countries the standard practice is to use a ‘Leach Field’ or ‘Soakaway’. This is usually a series of perforated pipes connected to the effluent and buried in the soil. The wastewater is then slowly filtered and any remaining pathogens have time to be digested by bacteria in the soil. This probably makes more sense in a temperate climate where cold temperatures can inhibit bacteria growth.

My feeling is that in the humid tropics we probably get away without a leach field because as soon as the treated effluent is in the open, the bacteria have everything they need (oxygen, heat, water) to very quickly demolish any remaining pathogens. In the context of a rainforest camp I cautiously suggest that I am happy with this approach, as long as the effluent is downstream from the camp. The rainforest is very good at decomposing organic waste.

Where I am not comfortable with this approach is in slightly higher density projects such as a field station or jungle lodge. Although the environment may not be bothered by treated effluent, I have to consider the possibility of it coming into contact with people. In this case a good alternative is to use a reed bed or maturation pond for the final stage of effluent treatment. In extreme cases of a lot of people on a very small site – an island resort for example – I was interested to read that a hydroponicum can be used for this purpose.

After researching all of this I am still left with one very significant question. How do you deal with the sludge? Clearly I do not consider it acceptable to send someone down to dig it out.

One idea I have been thinking of is to try to using alternating septic tanks. Once one tank is full the influent is re-plumbed to an empty adjacent tank. Extra organic matter (rice husks, oil palm fibres etc) is then added to the full tank to soak up any remaining liquid and then it is sealed off and left for a year or more. By the time the second tank is full perhaps the first tank would have digested to something relatively harmless which could be more safely dug out?

The problem I have with this idea is that unlike a composting toilet, the sealed off tank would not be able to be dried out thoroughly. Unless it could be dried out I am not sure that the sludge could be composted to something non-hazardous.

Another idea I’m thinking of follows the ‘seal it up and forget about it’ philosophy. The catch with this is that plastic and concrete do not just go away. I am wondering therefore whether it is possible to make a septic tank out of a biodegradable material? Obviously it would need to last long enough for the tank to fill up (say 2-5 years) but then after that it could be allowed to degrade, say over a period of 10 years.

There are two other unfinished leads in this story. The first is how a composting toilet works and the second is what happens to the sludge once it has been ‘honey dipped’? Those stories are for another day. Meanwhile if anyone has any helpful advice on the above I’d be very happy to hear it.

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