Anaerobic Treatment Technology

Wastewater treatment is based on the biochemical processes of methanogenesis (anaerobic methane fermentation) during the transformation of organic matter by a symbiotic association of microorganisms.

The main advantages of anaerobic treatment biotechnology are:
— the treatment of highly concentrated dissolved organic compounds ranging from 5,000 to 150,000 mg O₂/dm³;
— the utilisation of ‘heavy, solid’ organic matter (wood shavings, oilseed cake, pulp, suspended solids);
— conversion of up to 90 per cent of wastewater pollutants into biogas and only up to 10 per cent into anaerobic sludge biomass;
— accumulation of anaerobic sludge biomass – in powder form or granulated, depending on the type of digester, the characteristics of the wastewater, the organisation of its feed and other technological parameters of the process.

The main equipment for anaerobic treatment consists of sealed digester reactors with a capacity of up to 10,000 m³.
Conventional and industrial digester designs treat effluent containing suspended solids and fine-particle organic matter.

Removal of organic and inorganic mechanical impurities
ranging from 0.01 to 100 mm, sand, household waste, fibrous matter, etc.,
from the effluent. At this stage, mechanical phase separation and settling are employed.

The stage involving the separation of dispersed and emulsified particles from wastewater, as well as soluble and saturated toxic gases and volatile by-products.
This involves preliminary treatment of the effluent by pressure flotation and the treatment of suspended solids with chemical reagents – coagulants and flocculants – to facilitate sedimentation processes.

The use of additional feedstock increases biogas yield and is necessary when other organic production waste needs to be utilised. The feedstock is shredded and mixed to the appropriate concentration with the prepared liquid effluent or the recirculation flow from the anaerobic reactor – the digester. Preparing the mixture of raw materials (substrate) improves the mixing of biomass during methanogenesis, prevents stratification and floating in the reactors, reduces foaming, shortens the fermentation time, stimulates biogas synthesis and improves its energy quality.

The stage of biotransformation of pollutants in the effluent or substrate into treated effluent – digestate, biogas and active sludge biomass.
Treatment efficiency – up to 80% for BOD and up to 85% for COD.
The technical solution for the equipment is determined based on an analysis of the composition of the influent and the specified objectives for effluent treatment and biogas production:
conventional digester
— UASB
— 2UASB
— IC reactor

Methanogenesis produces ‘raw’ untreated biogas, which, like the digestate, is fed into an anaerobic digestion reactor for further treatment.

The stage of further fermentation of residual organic matter in the digestate to produce biogas is achieved through:

— prolonging the period of methane fermentation of the effluent in post-digester tanks;
— increasing the temperature;
— intensifying mixing.

This stage is an additional process and is only appropriate for the digestion of highly concentrated effluents and other insoluble types of organic production waste.
It enables the recovery rate to be increased by 10–15 per cent.
The digestate from the digestion reactor is fed to the decanting and purification stages.
In the digester, biogas accumulates beneath the dome membrane, which regulates the pressure; it undergoes an initial phase of hydrogen sulphide removal on a special sulphide-reduction screen and is conveyed via a gas pipeline to the purification stage.

The stage involving the separation and further treatment of digestate effluent and the minimisation of waste from local treatment plants.
This is achieved by removing 60–70% of the residual suspended solids from the methane slurry using thickeners and screw dehydrators.
At the same time, the liquid (decantate) and solid fractions of the digestate (cake) are separated.
The dewatered solid cake (SR 25%) is shipped as a solid organic biofertiliser.
The supernatant is clarified in a flotation unit and a lamellar clarifier, removing 90–95% of the associated suspended solids.
The clarified supernatant can be used as a liquid organic biofertiliser.
The treatment residue – flotation sludge – is the most active biomass and is returned to the anaerobic reactor.
Where necessary, pasteurisation and drying are used to disinfect the concentrated and solid fractions.

‘Raw’, untreated biogas contains up to 2,000 ppm of hydrogen sulphide and, as a corrosive component, causes corrosion of boiler and cogeneration equipment.
Preliminary purification of biogas is possible in a digestion reactor using a sulphur-reducing support mesh containing sulphur-oxidising bacteria of the genus Thiobacillus. This solution removes 50% of the hydrogen sulphide from the biogas.

Subsequent, more thorough purification is carried out using:
— a chemical wet scrubber-desulphuriser via adsorption using a 4% caustic soda solution, or
— a bioscrubber with immobilised sulphur-oxidising bacteria.

At the same time, excess moisture is removed from the biogas. Carbon filters are used for a more thorough level of post-treatment. The purified biogas has a moisture content of 60% and a residual hydrogen sulphide level of 50–100 ppm, which allows it to be used as fuel in boilers and combined heat and power plants.

Purified biogas is fed into a biogas-fired water-heating boiler to be converted into industrial heat.
Where biogas production volumes are significant, cogeneration units are installed to convert it into electrical and thermal energy.
Part of the thermal energy is used to maintain temperature conditions in anaerobic reactors and to meet other requirements of the plant.
The electricity generated is the end product of ‘green energy’ and is fed into the electricity grid via a transformer substation.
Any surplus biogas is utilised in an emergency flare.