Abstract
The anaerobic digestion of wastewater from the cleaning of tank cars transporting food and fodder was investigated in both bench and pilot scales with a single-stage, mesophilic (39 °C), completely mixed process. The promising results lead to the planning and building of a 1200-m3 full-scale biogas plant at TS-Clean cleaning station in Fahrbinde, Germany. Due to softened water used in the cleaning of the car tanks, the alkalinity in the digester decreased as predicted by the physicochemical model developed for this treatment process. The model showed that 2.4 kg NaHCO3/m3 of wastewater has to be added in order to control digester pH at 7.2 and to maintain the digester alkalinity at 3.1 g CaCO3/L. In a laboratory study, the decrease of alkalinity caused a volatile organic acids accumulation and pH drop below the optimal range. In this case, if chemical buffering was not added into the digester, the digester deteriorated. In a 3-year investigation, we confirmed that the strongly polluted WW from the cleaning of tank cars transporting food and fodder is suitable for an anaerobic treatment if the organic loading rate is controlled below 4 kg COD/m3/day, digester alkalinity is adjusted by NaHCO3, and micronutrients are added despite constant considerable variations in strength and composition of the wastewater. A biogas yield of 35–45 m3 CH4/m3 of wastewater and a COD elimination of 80–90% were achieved in bench- and pilot-scale experiments and are achieved in the full-scale biogas plant. The full-scale biogas plant is working stable with a biogas yield of 68 m3 biogas/m3 of wastewater.
Highlights
Food and fodder like chocolate, cacao, sugar, milk, fruit juice, starch, different types of oils, and glycerol in the form of liquid, paste, or powder are transported in tank cars
chemical oxygen demand (COD) degradation increased steadily to 80% in digester B11 and stayed stable at that level with initial OLR being reduced after 6 days from 6.8 to 3.5 kg COD/m3/day
Based on the experimental data and the model calculations, a 1200-m3 biogas plant was built in Total solids (TS)-Clean cleaning station Fahrbinde for treating the highly polluted WW from the cleaning of tank cars
Summary
Food and fodder like chocolate, cacao, sugar, milk, fruit juice, starch, different types of oils, and glycerol in the form of liquid, paste, or powder are transported in tank cars. The cleaning process consists in case of a strong pollution of the tanks in mechanical removal of remains and/or precleaning with 160 °C steam. All tanks are cleaned with 85 °C hot soapy water and are rinsed with water. In all cleaning processes, softened water is used. Pre-cleaning and washing of considerably polluted tanks generate strongly polluted wastewater. Washing of only moderately polluted tanks and rinsing generates moderately polluted wastewater. The strongly and the moderately polluted wastewater (WW) is mostly not collected separately and are often discharged to the communal wastewater treatment plant (WWTP) without any pre-treatment. The wastewater is pretreated in order to meet the standards for indirect discharge.
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