Experimental set-up and methods

This section is sub-divided into two sections, the first dealing with the batch digestibility tests used in developing the synthetic sludge and the second concentrating on the semi-continuous laboratory- scale digester tests.

5.1.1 Digestibility batch tests

During the development of the synthetic sludge, digestibility batch tests were carried out with digested sludge from Kidderminster STW as the inoculant and a range of synthetic sludges used as the feed. In order to minimise differences in the biomass content of the inoculant, the digested sludge was sieved to remove large lumps of biomass before it was added to the serum bottles. 30 ml of inoculant sludge was added to 70 ml of each type of synthetic sludge and placed in a 120 ml serum bottle. Tests were carried out in triplicate for each sludge and for the control which used a feed of deionised water. The bottles were sealed with a butyl rubber septum and an aluminium crimp and

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kept at a constant temperature of 35 °C. The tests ended when the biogas production subsided to the base level of the non-feed control, which took approximately 13 days.

5.1.1.1 Biogas volume measurement

Biogas accumulated in the 20 ml headspace of the bottles and the pressure of gas was measured using a manometer and then converted into a volume using the ideal gas equation:

PV=nRT Equation 5-1

where P is pressure, V is volume, n is number of moles, R is the Universal Gas Constant and T is temperature.

The final biogas volumes reported were based on standard temperature and pressure, 25 °C and 1 atmosphere (101325 N/m2) respectively. In order to produce an accurate curve of biogas production, biogas volumes were measured several times a day during the initial stages but this was reduced to once a day as biogas production slowed. Gas was measured by piercing the septum with a needle connected to a tube, in turn connected to a manometer. Fine, short needles were used to ensure that the needle tip did not touch the sludge and that the pierced holes in the septum would reseal easily. Needles were replaced when they became blunt.

5.1.2 Semi-continuous laboratory-scale digester tests

This section describes the apparatus and feeding regime for the semi-continuous laboratory-scale digesters, the storage and preparation of the feed and the methods for biogas collection and analysis.

5.1.2.1 Apparatus and feeding regime

Four identical 6-litre digesters were used for semi-continuous tests, as shown in Figure 5.1. They were heated to 36 ± 1 °C by means of water jackets, fed by a circulating heater, and the temperature of the digestate was monitored with a thermometer placed in the digester. Additionally, the

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digesters were insulated using reflective radiator lining. Each digester was equipped with a 4-blade paddle for stirring, positioned 50 mm above the base of the reactor, and four baffles around the walls of the digester. The digesters were fed with 250 ml of feed five times per week (Monday to Friday). The retention time was affected by the fact that the digesters were only fed on weekdays, as shown in Table 5.1. Ten litres of synthetic sludge were made up every other week. The digester feed composition was developed during the course of the research as a synthetic feed source for the laboratory digestion experiments and further details of its composition, rheology and nutritional properties are given in Sections 6.1, 6.3, and 6.4 respectively.

Table 5.1 - Digester feeding regime

Parameter Daily feeding Monday to Friday feeding Concentration of VS in feed 28.5 g/l

Daily feed volume 179 ml 250 ml

Actual retention time 33.6 days 24 days

VS added per feed 5.1 g 7.1 g

Organic loading rate (OLR) 0.85 g/l/d 1.18 g/l/d

5.1.2.2 Sludge sampling

Sludge samples were taken daily immediately prior to digester feeding. They were collected using the side arm of the digester and each sample consisted of 250 ml of sludge. If less sludge was required for analysis, any additional sludge was discarded. As far as possible, analysis was carried out on the sludge immediately after sampling. Where this was not possible, sample storage is detailed in the section relating to the relevant analysis.

107 Figure 5.1 - Lab-scale digester

108 5.1.2.3 Biogas collection

The volume of biogas produced by the digesters was measured using a semi-continuous gas flow meter. This was built to a design reported by the Water Research Council (WRC, 1975) and works by means of an inverted tipping bucket mechanism immersed in liquid. As the gas bubbles fill the bucket, it tips and a magnet activates a switch connected to a counting device. The flow meter used had a nominal bucket volume of 20 ml per count and was designed to measure gas flow rates of 1 to 20 l day-1. A more precise bucket volume was calculated by bubbling air through the gas flow meter and collecting it in an upturned measuring cylinder in a water bath. The barrier solution in the flow meter was acidified 75 % sodium chloride solution to prevent absorption of carbon dioxide and preserve the composition and volume of the biogas (Walker et al., 2009). A three-way valve at the top of each digester allowed venting, collection or sampling to take place. Gas was assumed to be at ambient air pressure when it passed through the gas flow meter. The final biogas volumes reported were based on standard temperature and pressure, 25 °C and 1 atmosphere (101325 N/m2) respectively and the volumes were converted using semi-continuous air pressure data collected from Birmingham Airport (UKMeteorologicalOffice, 2012). It was assumed that the collected gas was still at the temperature of the digester when its volume was measured.

5.1.2.4 Biogas analysis

Biogas composition was analysed daily whenever possible, using a handheld combustible gas meter (GMI GT44). The gas meter was calibrated by injecting five standard gas mixtures: 1) 100 % CH4, 0 %

CO2; 2) 75 % CH4, 25 % CO2; 3) 50 % CH4, 50 % CO2; 4) 25 % CH4, 75 % CO2; 5) 0 % CH4, 100 % CO2. A

1 ml plastic syringe attached to a needle was used for sampling gas from ports at the top of each digester. The samples were injected into the gas meter and the % LEL reading recorded. The percentage methane was then calculated from the calibration curve. Samples were taken in triplicate and a mean of the three points was recorded.

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