Pilot-scale installation for sludge fermentation in Leszno

The Water Utility Company in the city of Leszno, Poland, is the operator of a mechanical and biological wastewater treatment plant with nutrient removal for approx. 100000 PE. Investment objectives for the coming years include the extension of the treatment plant with facilities and installations enabling sludge utilisation for biogas production. A very important factor that influences the efficiency of the treatment plant operation and determines the final operating costs is the efficiency and stability of the fermentation process. Extensive domestic and foreign experience confirms the possibility of increasing biogas production by conducting a process of co-fermentation of sewage sludge and selected biodegradable waste from external suppliers.

In order to define the optimal quality and quantity of sludge and biodegradable industrial waste and wastewater to be used in the co-fermentation, the treatment plant operator invested in a pilot-scale installation for co-fermentation. By conducting the micro-scale fermentation process in laboratory conditions, it will be possible to reflect the process taking place in the fermentation chambers and forecast actual fermentation conditions.

The presence of such an installation will allow the fermentation process to be optimised by testing its resistance to interference, determining the susceptibility and fermentation efficiency of individual substrates, pre-forecasting the amount of biogas and methane produced, and preventing process inhibition as a result of early detection of adverse changes. This will allow the selection of substrates at the industrial waste and wastewater supplier, which is expected to result in improvements in the quality and stability of wastewater supplied to the treatment plant.

Components installed in the solution

The main elements of the pilot-scale installation are two glass bioreactors with a working capacity of 15 l each. The bioreactors consist of jacketed glass kettles with bottom closure (without a dead space). They are made of SIMAX borosilicate glass. The kettles are equipped with glass covers that are also made from SIMAX glass, with the required amount of process nozzles.

Both bioreactors are equipped with agitators with a spatula and a propeller, made of AISI 316 steel and powered by electric variable speed drive Type Hei-TORQUE – manufactured by HEIDOLPH. The agitators are guided using suitable guides with vacuum sealing – manufactured by COWIE.

The entire above-mentioned bioreactors have been incorporated in one common rack made of AISI 304 steel pipes with a diameter of 33.7 x 2 mm, with polished surfaces and Kee Clamp connectors – manufactured by MEGAN, Dimensions of the common rack: 1010 x 1150 x 1200/2000 mm.

Waste and sludge homogenizer, type IKA T 25, digital LR with S 25 N-25G-ST end-effector.

Two circulation thermostats Type Tip 100-2, heating power 2500 W, for feeding the heating medium to the coats of glass bioreactors and maintaining a stable temperature of the bioreactor input within the set values of 30 to 60 °C.

Meters as components of the installation: Biogas composition (CH4, CO2, O2) and flow are measured, as well as pH and T of the sludge.

A manual measuring kit which includes all necessary laboratory reagents and accessories,  is ready for the determination of the following parameters:

  • PH reaction
  • Temperature
  • Contents of volatile fatty acids (FOS, VFA, SCFA)
  • Alkalinity (TAC, ALK, OWN)
  • Determination of the FOS/TAC ratio

This allows for early detection of adverse changes in the fermentation process (even before the pH drops), thanks to which the operator will be able to take appropriate preventive measures protecting against the collapse of the process and serious financial losses in the event of stoppage or reduction of biogas production.

Operational mode

The installation is used to carry out the methane fermentation process for substrates originating mainly from the agri-food industry as well as to determine the efficiency of obtaining energy from organic waste. Both bioreactors in the installation are meant to operate in parallel – one as the reference, and the other as the test unit prepared in various compositions of the biomass input. Bioreactors for anaerobic fermentation with a biogas composition analyser are intended for periodic operation.

The efficiency of the biogas production will be measured using meters permanently installed at the reactors, while biogas composition will be analysed periodically/cyclically using a single portable analyser installed to the biogas discharge hose. Then, during the measurement of CO2 and CH4 concentrations, the gas will be directed through the flow rate meter to the composition analyser. The biogas produced, after the measurement of its flow and periodic analysis of its composition, will not be stored and will be released outside the test room.

After the end of the anaerobic fermentation test cycle, the exhausted material is removed from the inside of the reactor by opening the bottom valve and introducing a stream of clean water through the input nozzle into the reactor, with a slight overpressure obtained using a dedicated “gun”. At this time, the reactor’s content should be intensively mixed during its removal, with the administration of water until the reactor’s inside is thoroughly cleaned.

Before the co-fermentation pilot was purchased a research on waste market of the Leszno area was carried out in order to find out whether there is waste available in sufficient quantities to safely implement the process of the co-fermentation of sewage sludge and industrial waste. In addition to the market research, laboratory tests were carried out using different waste from voluntary industrial producers in the area. According to the laboratory tests, the recommended substrate to be used in the co-fermentation process is waste containing fats. It is not recommended to use aerobically stabilized sewage sludge as a substrate in the co-fermentation process.

The implementation of the co-fermentation process in the wastewater treatment plant in Henrykowo may generate several hundreds of zlotys of operating profit per year. The profitability of the process is influenced by costs of electricity -higher energy costs mean greater savings because of self-produced energy (observed trend of rising energy prices), actual costs of the final disposal of sewage sludge -higher disposal costs reduce operating profit from co-digestion (an upward trend in the prices of sludge disposal services is observed), final arrangements between the producer and the recipient concerning the commercial conditions for the transfer of waste (which party bears the transport costs, whether the acceptance of waste involves charging the producer or vice versa – the recipient pays for the waste as for “raw material” for the energy production).

Estimated operating costs of the installation are 560 EUR/year.

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