Increasing the energy efficiency of pig farm effluent by adding zeolite for biogas production in the thermophilic range – PhD thesis
Doctoral Thesis
THOMA A. KOTSOPOULOU, Graduate in Agricultural Science, Graduate in Civil Engineering, MSc, IKY Scholarship Holder
SUPERVISOR: Prof. Gerasimos Martzopoulos
This study investigated the effect of adding different quantities of natural zeolite on the anaerobic degradation of pig farm effluent. In particular, the effect of adding zeolite on increasing the energy yield of the effluent (greater quantity of methane, energy yield over a short period of time) was examined. Furthermore, the impact of introducing zeolite on the reduction of solids, volatile solids and BOD5 (environmental factors) was investigated.
Two preliminary measurements were carried out, which served as a guide for the design of the main experiment. Each preliminary measurement lasted 30 days. Specifically, the results of the preliminary measurements determined the quantities of zeolite used in the main experiment, as well as the duration of each replicate.
The zeolite doses used in the main experiment were 0, 4, 8 and 12 g of zeolite per litre of wastewater. The main experiment comprised 5 temporal replicates. Based on the results of the preliminary measurements, the duration of each replicate was set at 15 days. In each replicate, the experimental design was a fully randomised design without groups, with an equal number of samples. There were 4 treatments, and 2 samples were used for each treatment. The reactors used were stirred batch reactors.
During the experiment, the temperature was maintained at 55 °C and measurements were taken for the following parameters: pH, biochemical oxygen demand (BOD₅), total solids (TS) and volatile solids (VS), as well as ammoniacal and organic nitrogen. In addition, the quantity of biogas produced was measured daily. Measurements relating to the qualitative composition of the biogas were taken at the end of each replicate.
The following general conclusions emerged from this research:
1. The addition of zeolite resulted in an increase in the energy yield of pig farm effluent for biogas production in the thermophilic range. In the treatments where 8 and 12 g of zeolite per litre of effluent were used, the increase in biogas production was 65 per cent and 49.8 per cent, respectively, compared with the treatment where no zeolite was used. Methane production in the treatment using 8 g of zeolite per litre of wastewater amounted to 306 litres of methane per kilogram of volatile solids, in the treatment using 12 g of zeolite, it amounted to 297 litres of methane per kilogram of volatile solids, whilst in the treatment without zeolite, methane production amounted to just 186 litres of methane per kilogram of volatile solids. Energy was recovered in a shorter time in the treatments where zeolite was used.
2. The addition of zeolite reduced total solids to a statistically significant extent. The reduction was such that the treated effluent could be discharged into soil receptacles without further treatment (Ministerial Decision Y1b/2000, Government Gazette 343/95).
3. The reduction in BOD5 was satisfactory, exceeding 60 per cent in some cases. However, the final BOD5 concentrations remain too high for the effluent to be discharged into water bodies, although they are close to the limits set by the state for soil receptacles (Ministerial Decision Y1b/2000, Government Gazette 343/95).
The above results were obtained by adding zeolite, which appears to adsorb ammonia, thereby affecting both the toxicity of the ammonia and the C/N ratio, as well as maintaining the acid-base balance (pH) of the effluent.