Inexcusable for IWK to be ignorant of the functioning of a Septic Tank

Many of Encik Shaik Abbas lbrahim's statements in his letter "Importance of an efficient household septic tank" (NST, 20 Feb, 1996) are contradictory, misleading and technically incorrect. I am not an expert in sewage treatment, but as an engineer, I have been trained to be critical of statements laid before me that are illogical and inaccurate.

Right from the start, Shaik Abbas has pointed out that the scum layer of grease and oil prevents the absorption of oxygen in the sewage so as to promote anaerobic decomposition of organic matter in a septic tank.

This statement may be correct, but a septic tank is not intended to perform with a scum of grease and oil. Moreover, this scum layer is not present in all septic tanks. In domestic sewage, one must assume that grease and oil can only derive from kitchen wastes.

However, many septic tanks in Malaysia are designed to only treat wastes from the water closets while those from washing and kitchen are discharged directly into open drains. Under such circumstance and with the absence of the scum layer of grease and oil, does it mean that anaerobic activity can not take place in a septic tank?

The truth is, because of the confined and completely covered space within the septic tank, anaerobic decomposition will take place with or without the scum layer of grease and oil. Shaik Abbas also advocates that septic tanks should be well ventilated to prevent the accumulation of gases which could cause an explosion. One wonders, out of must be aware that household septic tanks are not fitted with air vents and therefore, according to his contention, they are potential time bombs!

If one were to examine the design of a typical septic tank one can see that though it is completely enclosed, its inlet and outlet are still connected indirectly to the atmosphere. Accumulated gases can escape through manhole covers, if they are not airtight, or through the outlet pipe to the filter.


Additional ventilation would not be beneficial to the process of anaerobic decomposition in the septic tank. Without a scum of grease and oil, well ventilated tank could function as a facultative tank where bacterial reactions both aerobic and anaerobic decomposition take place. There is nothing wrong with this except that the septic tank has not been specifically designed as a facultative tank.

Shaik Abbas also contents that sludge would accumulate at a rate of 0.2 cubic-meter per year per household occupant and therefore, with a household consisting of 5 occupants and having a septic tank of 2 cubic-meter capacity, desludging should be carried out once every 2 years. First and foremost, have studies been done by any competent authorities here or elsewhere to substantiate this rate of sludge accumulation? Or is it a figure simply plucked from the air to suit IWK's convenience?

If at all that this rate of sludge accumulation is justifiable, then a septic tank should be desludged more often than just once in 2 years. One can easily deduce that after one year of operation, the tank would lose half of its capacity and in two years it would be completely filled with sludge that it would not be able to function as a sewage treatment plant!

It is opportune at this juncture to point out that the ideal conditions for anaerobic decomposition of organic matter to take place are high organic strength of raw sewage with abundance of biological nutrients, high temperatures and the absence of toxic materials. Under these conditions, anaerobic treatment of sewage would result in low production of biological sludge. It is because of this reason that septic tanks are designed to cater for only wastes from water closets and exclude wastes from washing and kitchen.

Too frequent desludging of a septic tank may not promote its efficient performance. After desludging, it is a correct practice to leave a small amount of sludge in the tank to ensure adequate bacterial seeding to continue anaerobic activity.

Even with this practice, it may take some time to get the process fully operational. However, the advice from Shaik Abbas to also fill the tank with water after desludging is totally irrational as diluted sewage and the presence of dissolved oxygen would farther retard the process of anaerobic decomposition. If a septic tank is designed for a retention period of 24-hour, it would simply be filled up with sewage in a day! So, why the need to fill it up with water first?

It is interesting to note that all previous writers of this subject, including Shaik Abbas, have deliberately ignored the importance of the proper provision and functioning of the filter bed / soak-away. This constitutes an important element in sewage treatment using a septic tank. It has been reported that up to 50% of the removal of BOD and suspended solids of domestic sewage takes place in the filter.

A soak-away consists of a bed of granular stones laid in a trench to receive effluent from the septic tank. This effluent, which is partially treated, is distributed through a perforated pipe evenly over the trench. In this soak-away, further organic decomposition takes place in a aerobic facultative environment and the resultant effluent seeps down into the soil and finds its way to the nearest waterway.

This soak-away functions well in ground with good permeability and low water table. However, due to organic overloading , insufficient hydraulic flows and plugging of passages in between the filter media it could result in reduced treatment efficiency and foul odours. Therefore, to ensure proper functioning of the filter, rehabilitation by flushing or purging has to be carried out as and when necessary.

It is rather strange and incomprehensible to note that the privatisation of the national sewerage services does not seem to cover this aspect of biological treatment of effluent from a septic tank. If, for whatever reason that this part of the treatment is ignored, then the part of the privatisation contract on septic tanks would be grossly incomplete and would leave much to be desired.

Shaik Abbas has claimed in his letter that poorly maintained septic tanks are among the main causes of faecal bacterial pollution in our rivers. However, without in depth studies I doubt he could produce any statistics or data to substantiate his statement. This is a typical approach of IWK in making wild and sweeping statements in their desperate attempt to convince the public of the importance of proper sewage treatment.

I would suggest that IWK concentrate on those remaining 80% of the treatment plants that are still not functioning. These malfuctioning sewage treatment plants are a serious source of pollution to our rivers.

Lastly, I feel sad to have to write this letter to refute the misrepresentations by a senior member of IWK. They are supposed to be the custodian of all the sewerage facilities in this country and are to improve the environment and health through the proper implementation and maintenance of these facilities. Yet, from the various statements made, it appears that they are not even vaguely conversant with the functioning of the "simplest forms of sewage treatment first developed by the French in 1860s"!

read En. Shaik Abbas' letter here 2008