Anaerobic sewage treatment in the foodstuffs industry

Because of the large proportion of COD (chemical oxygen demand) in their sewage, many larger companies in the foodstuffs industry have their own biological sewage treatment plants, which are mostly operated on the traditional aerobic basis. These plants often require a lot of energy for aeration and mixing. They also produce carbon dioxide as an end product, together with large quantities of sewage sludge, which today must be disposed of expensively (50–90 € per tonne).

Modern anaerobic technology offers an alternative avoiding these disadvantages. Anaerobic processes rather involve the formation of biogas, a regenerative energy carrier, and a reduction in the amount of sludge by a factor of ten.

We would be very pleased to analyze the sewage accumulating in your company and find the best suited technology in economical and ecological terms.

Retention of biomass

A characteristic of anaerobic conversion is the low increase in the biomass, because most of the energy present in the sewage components goes into the end product methane and is therefore not available for production of biomass. In a research project for a company in the milk-processing industry, the task was for the sewage to be treated at the ambient temperature which, particularly in the winter, reduces the conversion rates and the formation of biomass. So that high conversions can be achieved, the biomass must therefore be retained in the reactor and concentrated. This can be done in two ways: By immobilizing the biomass on a carrier material, e.g. in a fixed-bed reactor, or by mechanical retention, e.g. by filtration in a membrane filter reactor.

Fixed-bed reactor with periodical circulation

The fixed-bed reactor (Fig. 1) contains a mineral aggregate (fixed bed) to hold back the biomass. During the passage, the organic components of the sewage are converted by the immobilized biomass. This means that there is a tubular reactor characteristic. However, the gradual increase of biomass normally causes blockage of the bed and channel formation, which over time reduces the conversion rate. In the case in question, the fixed-bed can be periodically circulated via a central delivery pipe and the special shape of the reactor, and thereby freed again from surplus biomass. This enables the conversion to be kept constant over long periods.

Membrane filter reactor

The membrane filter reactor (Fig. 2) is a reactor in which all mixing takes place via gas circulation. A new type of rotating disk filter is connected to this, which holds back the biomass and produces a biomass-free permeate as the main discharge flow. The rotating disk filter is a continuously operable, energy-optimized filtration unit developed at Fraunhofer IGB with a ceramic separating surface and long filter service lives. The biomass concentration in the system can be kept constant by the additional removal of a small amount of the reactor content.

Experimental results

Test operation of the two reactors on a technical scale has shown that the conversion rates are much higher than is the case with the aerobic system in use with the industrial partner. This has a sewage holding time of 6 days with a load of 0.5 kg COD/m3d. With our anaerobic test systems, we achieved a load of approximately 4 kg COD/m3d with a liquid holding time of approximately 12 hours, the COD removal being greater than 80 percent. The gas production rate is approximately 0.4 l of biogas / g CODremoved. Thanks to the biogas being used for energy via combined heat and power generation, systems of this type normally have a positive energy balance combined with cost savings resulting from the drastically reduced amount of sludge produced.

The fixed-bed circulation reactor has the advantage of a simple design, but no biomass-free discharge is achieved. Part of the biomass is substrate-limited by the immobilization as a result of the operating principle, and the scale-up is more difficult. The membrane filter reactor has a biomass-free discharge, the biomass concentration is easy to control and the scale-up is simple. It should be operated as a fully-mixed system with at least two stages.