Many times Bio digesters are confused with Sewage treatment & recycling plants, here is a brief comparative analysis.

WASTE WATER TREATMENT PLANT (WWTP) – Sequential Batch reactor (SBR)



The SBR waste water treatment process is an activated sludge process in which the sewage after pretreatment is introduced into a Reaction Tank (or SBR Tank), one batch at a time. Wastewater treatment is achieved by a timed sequence of operations which occur in the same SBR Tank, consisting of filling, reaction (aeration), settling, decanting, idling, and sludge wasting. Clean treated water is produced.

A biodigester is like a mechanical stomach. It is fed with organic material, which is broken down (decomposed) by micro-organisms (bacteria) is an oxygen-free (anaerobic) environment to produce a renewable energy called biogas (methane and carbon dioxide) and other material that is mainly used as fertilizer.

How does it work?

The various stages in the sequence are as follows:
Stage 1: Filling
The SBR Tank is filled with the influent wastewater. In order to maintain suitable F/M (food to microorganism) ratios
Stage 2: Reaction
This stage involves the utilization of biochemical oxygen demand (BOD) and ammonia nitrogen by microorganisms. The waste water is aerated with a blower t provide oxygen for microorganismas as well as kill odor
Stage 3: Settling
Sludge settles leaving clear, treated effluent above the sludge blanket.
Stage 4: Discharging
Clean treated effluent is removed from the tank through the discharge air sunction pipe without disturbing the settled sludge.
Stage 5: Sluge wasting & Idling
Excess sludge is pumped back to pretreatment and system waits idle until it is time to commence a new cycle with the filling stage.

The feed stock is shredded with water to a slurry and added to the digester on a regular basis. Within the closed tank methanisation occurs. Each day, the waste to be added needs to be mixed with water and/or ground to a liquid state. Kitchen scraps, such as banana and orange peels need to be ground in smaller parts which can be time consuming. Each day, the biodigester effluent needs to be removed manually from the effluent tank.The Methane gas produced too has to be released real time for consumption.


This process requires Oxygen as it is aerobic. Usually provided by a blower

Anaerobic process that does not require oxygen

Raw material

Black (from the toilets) & grey water (from kitchens, bathrooms, house cleaning)

Organic solids & black water only

Products of the process

Clean treated water reusable for secondary purposes e.g. irrigation & flushing toilets.
Sludge that requires to be exhausted after five years and can be used as fertilizer

Methane gas.
Slurry that has to be removed daily and can be used as fertiliser


Operate effectively in temperatures as low as 10 degrees

Operate effectively between 20 to 40 degrees


Not so much space required, and the system is underground

Requires large space to manage all the processes and by products