Desludging interval of two years an "estimate"

Many statements on the subject of septic tanks have been make by the Department of Sewerage Services (DSS), Indah Water Konsortum (IWK) and the concerned public. They are all centered on the need and frequency of desludging the tanks under the national sewerage services privatisation contract.

In recent statements made by DSS (NST Sept. 12) and IWK (NST Sept.16), both maintained that septic tanks were required to be desludged once in 2 years. The reason put forward was simply that it was a design requirement.

The assertion by DSS and IWK was not only weak and misleading but also could not be substantiated technically.

In Malaysia, planning and design of septic tanks are currently governed by:

(1) MS1228:1991, Malaysian Standard Code of Practice for the Design and Installation of Sewerage Systems, published by SIRIM and

(2) Guidelines for Developers on the Design and Installation of Sewerage Systems. Section 2 - Residential Housing and Mixed Developments, published by DSS in 1995.

The above two publications do not specify the frequency of desludging a septic tank nor the detention time of waste water in it. However, it is specified that the capacity of a septic tank is to be estimated based on an average daily flow of 225 liters per person and the number of persons served per residential premises is 5. There is, however, a proviso that the capacity of a septic tank should not be less than 2000 liters.

Using the design criteria as specified and assuming that a 24-hour detention time is reasonable, it can be easily deduced that:

(a) A 2000-liter septic tank serving a normal residential premises is oversized by about 2 times compared with the required capacity computed using a flow of 225 liters per capita (per person per day) and a detention time of 24 hours;

(b) A sewerage flow of 225 liters per capita is excessive considering that the potable water supply per capita to an urban area is of the similar order. Moreover, not all water used is discharged into a septic tank; and

(c) A conservatively designed septic tank would effectively give a bigger sludge storage capacity and a longer detention time available for anaerobic decomposition (biological action by bacteria in the absence of oxygen) of organic matters to take place which would result in a lower production of sludge.

The above deductions mean that a longer desludging frequency should be acceptable.

Furthermore, from the statements made by DSS and IWK, it is apparent that both had deliberately or otherwise ignored the existence of a filter bed which is part and parcel of a septic tank. A filter consists of a bed of granular stones and is normally bigger than the tank. The partially treated effluent from the tank passes to the filter bed where further biological decomposition takes place before the final effluent is discharged.

As far as treatment of sewage is concern, the filter is as important as the tank. It is reported that up to 50% of the removal of impurities (BOD and suspended solids) takes place in the filter. However, a filter can be choked and therefore requires periodic maintenance like flashing or purging. Proper and adequate maintenance of the filter is therefore as important as desludging the tank.

From all the above, it can be easily concluded that a desludging frequency of 2 years is at best a "guesstimate" and has on technical justification to be adopted as a design criterion. The frequency of desludging of a septic tank clearly depends on the organic loading, the rate of sludge formation and the storage capacity of the tank. Unless a study has been carried out under Malaysian conditions, any number of years quoted should only be considered as an estimate.


(The above article was published in NST, Sept 26, 1996) 2008