An IST comprises two chambers connected in a series. In the first chamber, solids from the incoming sewage settle forming a "sludge", while grease and oil float to the surface forming a "scum" layer. Effluent from between the scum and sludge layers then passes into the second chamber where further sedimentation occurs. Finally, the effluent leaves the second chamber and is discharged into a drain or allowed to percolate into the soil.
The sludge in the tank undergoes anaerobic digestion and is converted into more stable organic compounds and gases such as carbon dioxide (CO2), methane (CH4) and hydrogen sulfide (H2S). ISTs are usually designed for a 24-hour retention time. Enough storage capacity is provided so that scum and sludge can be deposited in the tank for up to two years after which it must be desludged to keep the tank operating satisfactorily.
ISTs are suitable for single dwellings or individual buildings with a population equivalent (PE) up to 150 and installed where there is no central sewerage systems and where effluent discharges will not adversely affect the environment. It is a cheap solution to disposing of sewage. However, ISTs only partially treat sewage and concentrated groups of tanks can overload the capacity of the receiving environment creating health and odour problems. There are currently over one million ISTs in Malaysia, making it by far the most common type of sewage treatment system.
Connected sewerage service customers have sewage outlets that are directly linked to a sewage treatment plant via an underground network of sewer pipes. IWK is responsible for the operations and maintenance of all public sewage treatment plants. Public sewage treatment plants are plants which have been handed over to the Government (National Water Services Commission) by the Developer.
Premise with connected sewerage service/system has one rectangular metal cover, which is called an inspection chamber and is usually located outside the premise's compound either at the side, rear or front. In the inspection chamber are sewage outlets from the premise's toilets, bathrooms and kitchen that are connected to the public sewerage pipelines to the public sewage treatment plant. If there is any blockage, the checking and clearing of the blockage will be done via the inspection chamber.
However, in the case of bungalow houses, there are usually more than one inspection chamber depending on the number of toilets and kitchen in the house. If the house has 5 toilets and one kitchen, there will be 6 inspection chambers located outside the premise's compound instead of just one inspection chamber. More often than not, owners of bungalow houses tend to think that the inspection chambers are septic tanks. (Septic tanks are usually arranged in series and not located in various places outside the premise's compound)
Another frequent confusion with this type of system is the differentiation between internal or private and public sewerage pipes. Many users are unaware of their ownership and responsibility to repair or replace broken pipes and remove clogged pipes between the inspection chamber and the public sewerage pipes or manhole at their own expense. IWK is mainly responsible for the maintenance of the public sewerage pipes.