Importance of an efficient household septic tank

by En. Shaik Abbas Ibrahim


I REFER to the several letters that have appeared over the last two months regarding the proper functioning of septic tanks.

Septic tanks are among the simplest forms of sewage treatment and were first developod by the French in the l860s. There are now more than 1.2 million septic tanks in Malaysia, making them by far the most common type of sewage-holding systems.

A typical septic tank comprises two chambers connected in series. In the first chamber, solid matter from the incoming sewage settles to form sludge while grease and oil float to the surface to form a scum layer. The scum layer prevents oxygen from dissolving in the sewage, causing the sewage and sludge to undergo mesophilic anaerobic digestion. During this reaction, the sludge and other solid matter are broken down into organic compounds and gases - namely carbon dioxide (CO2), methane (CH4) and hydrogen sulphide (H2S).

As such, a septic tank should also be well ventilated to prevent the accumulation of gas, which could cause an explosion. The sewage then passes into the second chamber where further sedimentation and anaerobic digestion occur. The partially-treated sewage is now discharged into the waterways through a network of drains or allowed to percolate into the soil.

As the sludge accumulates, the liquid volume between it and the scum layer reduces until no further sedimentation takes place. This liquid volume is critical in ensuring the sewage is retained for a sufficient period for the proper sedimentation of solids and anaerobic digestion to take place before the effluent is discharged into the waterways.

The minimum retention period for the proper treatment of sewage is at least 24 hours. In order to maintain this retention time, the design of a septic tank for a typical household with five persons should have a minimum capacity of two cubic metres, this is based on the fact that studies have shown that under normal circumstances, septic tank sludge accumulates at a rate of 0.2 cubic metres per year per person.

Hence, the average septic tank-holding capacity will overload in two years causing a decrease in the retention time of sewage, causing untreated sewage, scum and or sludge to be carried out of the septic tank into the waterways. This directly leads to the pollution of our waterways and is a health hazard.

To ensure one's septic tank keeps operating satisfactorily and in the interest of public health and the environment. the Department of Sewerage Services has directed Indah Water Konsortium to desludge all household septic tanks at least once in two years.

A new septic tank should be filled with clean water before using the system. For a normal system, the amount of grease coming from the kitchen would not be sufficient to warrant the installation of a grease trapprior to the septic tank.

AH detergents, disinfectants and other household cleaners do not unduly hinder the bacterial reactions. There is also no necessity for the addition of various commercially available preparations which are said to start, accelerate or assist bacterial action in the tank.

To date, these products do not have any proven track record. If odorous conditions should occur shortly after the initial use of a system, a cup of lime may be flushed down the pan each day until the odour abates. Septic tanks should not be washed or disinfected after desludging but simply refilled with water to reduce odours on start-up. A small amount of sludge is alwaysleft in the tank to re-establish bacterial action.

Mother Nature takes care of the rest. Poorly-maintained septic tanks discharging sub-standard effluents into the groundwater and / or the waterways are among the main causes for the high incidence of faecal bacteria pollution in our rivers.

About 72 per cent of our rivers are polluted to various levels and raw sewage has been identified as the main pollutant. Our water quality index has also been deteriorating at a rate of 1.2 per cent for the last decade.

As a part of the nationalisation of sewerage services in the nation, the Department of Sewerage Services will, where possible, gradually phase out septic tanks and instruct householders to connect them directly to suitable sewage treatment plants through a network of pipelines. This will ensure modern and efficient sewerage services befitting a nation embarking on industrialisation.

It is envisaged that by the end of the concession period, 85 per cent of the domestic households will be provided with connected services and the rest will be provided with septic tank services in 45 of the 144 local authorities that IWK will service.

In the remaining 95 local authorities, 30 per cent of the domestic households will be provided with connected services and 70 percent with septic tank services. The importance of a modern and efficient sewerage system is essential to ensure a cleaner and healthier environment.

Despite being a tropical country with abundant rain, our water resources are diminishing quickly. We are no longer in a position to take our water supplies for granted. If our waler-polluting habits are allowed to continue, there will come a time when good water supply will not be technologically feasible or financial affordable.

IWK encourages public feedback and requests that all inquiries relating to its services be directed to its Customer Service Centre via its toll-free service line 800-3495 for immediate atention.

Shalk Abbas lbrahim,
Senior general manager,
IWK 2008