Experimental set-up

For the determination of anaerobic biodegradability, batch tests were carried out. Pre-tests with blackwater according to a German standard method (Anonymous 1985) resulted in unreliable and hardly reproducible data. Thus a multiple flasks system was selected where several flasks of polyethylene (0.5 ml) represent one batch “reactor” (Sanders 2001). At the beginning, all flasks had the same content. It is assumed that the anaerobic processes are similar in all flasks. All flasks were closed, the biogas escaped through an airtight tube which purges in a water tub but was not collected. The multiple flask system was stored in a room at 30 °C. After definite durations, the “reactor” was sampled, three flasks were emptied, the content was analyzed and the concentrations were averaged.

Two series of batch tests were carried out:

Batch test serie B1 was carried out with homogenized blackwater only. Each time the “reactor”

was sampled (five times in defined intervals within 22 weeks) three flasks were emptied, the content was analyzed and the concentrations were averaged.

Batch test serie B2 was carried out with blackwater and inoculum sludge. Therefore a big quantity of blackwater and inoculum sludge from a mesophilic sewage sludge digester was mixed half/half and homogenised. Half of the flasks were filled with 400 ml of the mixture.

Similarly, another half of the flasks were filled with 400 ml inoculum only. Each time the

“reactor” was sampled (five times in defined intervals within 14 weeks) three flasks were emptied, the content was analyzed and the concentrations were averaged. Similarly, three flasks with inoculum sludge were emptied and analyzed. Due to the reference flasks with inoculum, the data for pure blackwater could be calculated.

CSTR experiments

At the Institute of Wastewater Management and Water Protection, the bench scale plant was installed (Figure 5). It consisted of three parallel continuous stirred tank reactors (CSTR). Made of PVC cylindrical material, each reactor has 10 l capacity corresponding to a 190 mm diameter and 355 mm height. The horseshoe mixer was actuated by an electric engine. The reactors were kept at mesophilic temperature (37 °C) in a tub filled with water. The water temperature in the tub was controlled by a thermostat. Water losses through evaporation were reduced by spherical plastic balls covering the water surface. The reactors were filled up to 8.5 l and 1.5 l was headspace.

All hose connections of the reactors were placed at the top. Firstly, the headspace of each reactor was connected to a gas counter via a flexible PVC hose (diameter 7.5 mm). In front of the gas counter, a gas mouse was placed enabling the sampling of biogas for composition analysis.

Secondly, a flexible PVC hose (diameter 7.5 mm) was connected at the top of the reactor to a tube with a length of 120 mm that was inserted from the top vertically down inside the reactor.

This hose was used for taking output samples from the reactor centre and feeding via an electrical peristaltic pump. Thirdly, each reactor was equipped with a flexible U-tube for pressure equilibration during feeding. Having an inside diameter of 38 mm the U-tube was partly filled with a sealing liquid of pH 2. This liquid (200 g sodium chloride and 5 g citric acid per 1 l deionized water) prevents the dissolution of CO2 from the biogas. Methyl orange indicator was added to check the acidity. Fourthly, at the top of the reactor a hole for a pH-electrode was plunged into the reactor which measured the pH value online. Due to an airtight sluice, the pH electrode could be checked and cleaned regularly. Each hose was equipped with a lab glass valve for maintenance and easy disconnection of parts of the installation. Most important was the water and airtight installation of the whole bench scale plant which was controlled regularly with the empty plants by pressure tests.

The daily amount of produced biogas was continuously measured by gas counter connected to each reactor. The gas counter was constructed based on the development of the Milligascounter by Scherer (2002). It works by liquid displacement and allows the precise measurement of small and fluctuating gas flows.

Figure 5: Scheme of the CSTR

The gas counter device consists of a cylindrical tube of PVC (diameter 135 mm and height 165 mm) and is filled with 1.5 l of the same sealing liquid as the U-tube. It is equipped with a chamber with two equal compartments made of acrylic glass which rotates via an axis when one compartment is filled with gas. The gas flow produces a flipping of the chamber which is detected via a magnetic signal. A sensor delivered the signal every minute to the data processing computer online. By means of online measurement of air temperature and barometric pressure, the produced biogas was recalculated continuously (every minute) for standard temperature and pressure (T = 0 °C, p = 1,013.25 mbar) according to Anonymous (1985).

Pre- and post-treatment

Pre-pasteurization of the feedstock was carried out according to the German organic waste ordinance (heating at 70 °C for one hour or 55 °C for 24 hours) (Anonymous 1998). Blackwater or mixture of blackwater and kitchen refuse was put in a pot on a hot plate, heated and mixed from time to time manually. After reaching 55 and 70 °C in the feedstock, the pot was put in a hot cabinet for one hour and 24 hours, respectively.

Pre-acidification: Additionally, investigations into acidification of the feedstock were carried out in terms of microbiological parameters. Therefore, the blackwater or the mixture of blackwater and kitchen refuse was divided in several polyethylene flasks. The flasks were closed and put at

Water Tub

constant temperature (20 or 37 °C) in a cabinet for up to six days without mixing. Upon each analysis, one flask was utilized (multiple flasks system).

Post-storage: From the digested output, four samples at different times were taken. Several 250 ml polyethylene flasks were filled up to 200 ml with the output. The flasks were closed but still allowing a gas exchange and stored at ambient temperature inside. The temperature varied seasonally from 15 to 27 °C. The storage time was up to 90 days. The multiple flask system was applied again for sampling.