Flexible approach

Anaerobic reactors have come a long way since the big, custom-built plants of the 1980s. Melanie Brown reports on a demonstrator project, showcasing the latest anaerobic treatment technology

A new, modular anaerobic treatment reactor is being demonstrated in a 2.5-year, £490,000 project supported by Bio-wise. The process, developed and designed by Clean Technologies, uses computer control to optimise reactor performance and minimise costs. The demonstration is being carried out at a malt extract factory in Newark, Nottinghamshire, owned by International Diamalt Company.

In the early 1980s, anaerobic reactors were usually large plants custom-designed and built on site. Clean Technology’s new modular digesters are a cross between a sludge blanket and an upflow packed reactor, and allow a higher rate of treatment than can be achieved using either process alone.

The pre-fabricated reactors are cheap and flexible. They can be installed within a few weeks and additional units can be added or subtracted to accommodate varying effluent flows. In particular, they allow treatment of low to medium volumes of high-strength food industry effluent.

On-site performance

The process converts the organic pollution load in the effluent (chemical oxygen demand or COD) in a series of stages into clean, high-calorific fuel gas, suitable for energy generation. Understanding the critical bacterial reactions involved in the anaerobic digestion of waste has allowed Clean Technologies to optimise conditions for conversion of organic load into biogas. The company’s software can control parameters such as pH and temperature very accurately. “The fine process control allows the operator to squeeze out the optimum performance from the reactor, making it highly cost-efficient,” explains managing director Paul Ditchfield.

A 150m3 anaerobic reactor module was installed at the Newark site in October 2001. It treats 300m3 of effluent per day, which contains 2,000-86,000mg/litre COD and 300-1,300mg/litre suspended solids.

Because anaerobic bacteria grow relatively slowly, seeding with biomass from other plants was used to reduce start-up time. The reactor is designed to convert 90% of the COD load into biogas, which means that sludge production from the plant is low. Sludge leaves the reactor as suspended solids in the final effluent and there is no requirement for sludge disposal to landfill. The final effluent is discharged to sewer.

The capacity of the reactor to generate gas depends on the composition of the effluent and can be determined in the laboratory prior to installation of a plant. Typically 40-45m3 of biogas (75% methane and 25% carbon dioxide) is produced per hour at the Newark site. This would generate more than 100kW of electricity. The biogas is currently flared, but will be used when an additional boiler is installed.

During the demonstration period, the reactor has received a number of shock loads due to process excursions, where COD increased from 10,000mg/litre to more than 80,000mg/litre. Gas production rose rapidly, demonstrating the capacity of the process to respond to changes in feed concentration. However, the pH control system took several hours to regain its optimum value and final effluent quality temporarily deteriorated, demonstrating that periods of under-performance can follow shock loads. This indicates that better control of influent COD is required for optimum performance of the process. This could be achieved by installing an additional balancing tank at the front-end.

Adding up the advantages

Initial cost assessments suggest that the capital cost of the process is half the price of alternative anaerobic treatment plants per unit of COD removed. Prior to installation of the plant, effluent from the malt extract factory was discharged directly to sewer, incurring charges under the Mogden formula. The treatment plant has the potential to reduce these charges by up to 80% – a considerable cost saving.

The trial continues until June and optimisation of the plant is still underway. The process is applicable to a wide range of industrial effluents including brewing, distilling, food processing and textile and paper production. In order to extend the range of applications for the process, Clean Technologies is planning to produce a smaller glass fibre reactor for low-volume effluent applications.

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