Evaluation of a biogas plant in Adeiso, Ghana

Abstract: The purpose of this study was to evaluate the function, stability and performance of the biogas plant at HPW Fresh & Dry Ltd. in Adeiso, Ghana. The study showed that the process was functioning, with an adequate average organic loading rate and methane content of the biogas. However, the digesters had low alkalinity and were sensitive to disturbances, and the uneven feeding caused considerable pH and biogas flow variations. The influent consisted of 47 % fruit waste and 53 % water. At eight times during the project period, poultry manure was added. The feeding occurred mainly between 8 am and 5 pm, ranging from 0 to 20 m3 per digester and day. The average daily feeding to one digester was 9.1 m3 which gives an organic loading rate varying between 0 and 5.5 kg ODM per m3 and day. The average value was 2.6 kg ODM per m3 and day. The methane content of the biogas varied between 39 % to 61 %, with an average of 51 % in Digester 1 and 50 % in Digester 2. The methane content decreased during the day due to the feeding. The H2S content was as high as 1900 ppm in the beginning of the period, probably due to accidental addition of sulphur containing preservatives to the digesters. The concentration decreased steadily as a result of process recovery. From the two digesters combined the total biogas flow was estimated to values between 24 and 59 m3/h. The flow increased generally during the day, which is explained by the feeding pattern. The pH of the influent varied between 3.6 and 5.2 with an average value of 4.0. The pH increased when poultry manure was added. The pH of the digestate was 6.6-7.1 with an average of 6.9 in both digesters. The temperature ranged from 31 to 34°C with an average of 33°C and was higher in the end of the period. Generally, the pH decreased during the day, due to the feeding pattern. On days with more feeding, the pH decreased more. The pH was higher and more stable in the end of the project period, which is due to higher alkalinity and/or higher temperature. The alkalinity was low, the bicarbonate alkalinity was measured weekly to values between 2 000 and 3 300 mg HCO3/l, while the total alkalinity ranged from 3 300 to 4 300 mg HCO3/l. However, the alkalinity increased during the period, probably due to the addition of poultry manure. The nutrient content in the substrates was low, giving a high C/N ratio. This implies a nitrogen shortage in the digesters. Also, the ash content in the poultry manure was high, making it desirable to replace it with another source of nitrogen. From the findings of the study recommendations on further development of the plant were made. These included a more even feeding, an improved mix of substrates and monitoring of operation parameters.