Anaerobic-aerobic treatment technology

This integrated process is also referred to as a complete, two-stage process, as the combination of aerobic and anaerobic biological methods yields the most effective wastewater treatment results, and the effluent from one process serves as the raw material—or substrate—for the other.

The method is used where there is a large volume of highly concentrated effluent – more than 8,000–10,000 mg O₂/dm³ in terms of COD – and to achieve a high degree of pollutant removal and transformation, with the final treated effluent discharged into water bodies.

The main phase is anaerobic methane digestion, whilst the additional, secondary phase involves further treatment in an aeration tank.

During the first, methanogenic stage, 80–90% of the pollutants in the effluent are removed, whilst the remainder is further treated using aerobic methods. Primary sludge and excess sludge from the aerobic stage can serve as a bonus substrate for biogas production during the methanogenic stage of the treatment process.

This integrated technology enables

– the effective removal of pollutants from wastewater with their subsequent utilisation,

– a reduction in energy consumption for treatment processes,

– the production of a renewable energy resource – biogas.

Where wastewater contains a large amount of organic pollutants, biogas can be utilised through cogeneration to produce electricity, which makes it possible not only to power the wastewater treatment process using its own energy resources, but also to generate revenue through the ‘green’ tariff.

The integrated anaerobic-aerobic technology involves the use of equipment for each of the stages.

Removal of domestic solids, sand and foreign impurities by means of mechanical phase separation and settling.

Removal of impurities using cyclones, settling tanks and sand traps.

An essential stage where domestic and other effluents containing lumpy contaminants are introduced into the flow.

Treatment with coagulants and flocculants to separate out suspended solids. The treatment process is based on flotation – a process whereby the effluent is supersaturated with air, and molecular forces cause the contaminants to agglomerate and concentrate, bringing them to the surface for subsequent removal. This ensures the removal of substances that may inhibit and reduce the efficiency of biological processes for the degradation of pollutants in subsequent stages.

Anaerobic treatment is the main stage of a comprehensive process that ensures the biotransformation of the majority of pollutants in wastewater or substrate into biogas, active sludge biomass and treated effluent – digestate. Treatment efficiency is up to 90% for BOD and up to 85% for COD. Depending on the composition of the influent or substrate and the objectives set for their treatment and biogas production, a technical solution is selected: a conventional digester, UASB, 2UASB, IC reactor or others. Methanogenesis results in the production of ‘raw’, untreated biogas. Where highly concentrated effluent or effluent containing insoluble organic waste is used, the resulting digestate is fed into an anaerobic digestion reactor for further treatment, additional gas production and degassing.

Further treatment of effluent following the main anaerobic stage to remove residual soluble organic compounds and biogenic mineral components.

The process is carried out by aerobic bacteria that grow and accumulate in activated sludge or in the biofilm on a moving carrier, thanks to their ability to immobilise on inert material.

Technical solution: a two-stage MBBR reactor with Aqwise AGAR moving-bed biomass carriers.

COD removal efficiency – 95%.

Effective nitrogen removal.

A 75% reduction in excess sludge compared to conventional aerated tanks.

A 20–30% reduction in energy consumption for aeration.

Further treatment is possible using IFAS and AS solutions utilising activated sludge in a suspended state.

This stage ensures the separation and minimisation of VOC waste volumes, as well as the further treatment of by-products for subsequent disposal or sale.

The concentration of ballast substances in effluents and sludge is initially carried out using flotation units, lamella clarifiers and sludge thickeners. Sludge thickening and separation of the solid fraction are carried out in dewatering units and press separators. To enhance the efficiency of the integrated effluent treatment process, sludge separated from the aerobic stage may be directed to the anaerobic stage to minimise the volume of secondary organic matter by converting it into biogas.

The removal of hydrogen sulphide from biogas takes place in stages, using a wet adsorption scrubber and carbon filters.

The biogas is dewatered using heat exchangers.

The purified biogas is used to generate electricity and heat in cogeneration units, or is burned in boilers to heat water in order to maintain the required temperature conditions for the main stages of purification.

A flare is used for emergency combustion of biogas.