Why does sludge need to be regularly collected from septic tanks, johkasou, etc.?

Wastewater – the water that has been used and goes down to sinks, bathtubs or toilets – is, for the households connected to a sewer network, conveyed to a wastewater treatment plant where it receives a treatment to be cleaned before discharge to the environment. For households that are not connected to a sewer network, wastewater is usually sent to an on-site wastewater treatment system, such as a septic tank for most countries or a Packaged Aerated Wastewater Treatment Plant (PAWTP) – so-called johkasou in Japan – usually buried near the house it serves and connected by a sewer pipe to the indoor plumbing. It can also be disposed of in a pit latrine or other types of facilities.

Wastewater contains two portions: a liquid and a solid portion. As Wastewater flows into the PAWTP or tank, the solid portion settles and gradually accumulates as sludge in the bottom while scum builds up on top of wastewater. If not regularly removed, the accumulated sludge and scum will fill up the PAWTP/tank and overflow with wastewater, in addition to not enabling the facility to perform as originally designed.

Unfortunately, in many countries sludge is not collected at regular intervals, which should be done when necessary, resulting in partly treated or untreated wastewater being discharged to the environment as well as overflowing sludge. This can cause negative impacts on the environment leading to the decrease of river water quality – eventually up to the death of aquatic life – and ultimately affect people’s health and the economy. Sludge discharge can also lead in solids accumulation in the drainage system, thus increasing the risk of flooding and associated health hazards. Therefore, sludge management – including regular collection, safe handling, transport and hygienic treatment – is a critical item to consider, develop and integrate in any sanitation plan for urban and rural areas.

Below: Sludge Management and Associated Impacts in Septic Tanks

Figure courtesy of IWK

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