8. Effluent from biodigesters with different retention times for primary production and feed of Tilapia (Oreochromis niloticus)
Authors: San Thy and Preston T R 2003
Livestock Research for Rural Development (15) 9 Retrieved, from
Summary: A completed randomized block design was used for allocation of two treatments to six ponds, arranged in three blocks. The treatments were: effluent from biodigesters with 10 (ERT10) and 30 (ERT30) day retention times. The experiment had a duration of 120 days from 1st July to 6th November 2002. The effluent was a mixture of pig manure and water with a solids (DM) concentration of 60 g/litre, which with hydraulic retention times of 10 and 30 days, was equivalent to a loading rate of 3.06 and 1.02 kg DM manure per m³ of liquid volume of the biodigester. The composition of the effluent for the hydraulic retention times of 10 and 30 days was total N content, 1003 and 1066 mg N/litre, ammonia-N 486 and 636 mg/litre, and ammonia-N to total nitrogen ratio, 0.50 and 0.60. The ponds were 2 x 3m and 1 m deep, and were lined with a cement and soil mixture to avoid water leakage through the sandy soil. Quick-lime (CaO) was applied to the bottoms of all ponds at the rate of 100 g/m², 10 days before stocking with fish. This liming was to eliminate parasites and pathogenic organisms and to increase the pH (Pich Sophin and Preston 2001). The ponds were filled with water 3 days after liming. The effluent was applied in quantities equivalent to 160 kg N/ha over the 120 days, equivalent to 0.133 g N /m2 /day. The effluent was taken from each biodigester immediately after charging with manure and water and was applied at intervals of three days. Each pond was stocked with Tilapia ( Oreochromis niloticus ) at a density of 2 fish/m². The fish were introduced as fingerlings about 3 to 7 cm length. Samples of effluent were taken before application to the fishpond every three days for determination of pH, DM, OM , N and Ammonia-N. The growth rate of the fish was determined by recording the length and weight every 20 days in the morning at 8:00am before loading the ponds with effluent. The fish were caught with a seine net and put in a small basket to measure the length and weight. The length from the tip of the mouth to the caudal fin was measured with a graduate ruler. At the end of the experiment the total fish biomass and the weight and length of each fish were recorded. The oxygen level of the pond water was measured every two days, two times during the day in the early morning at 6:00am and in the afternoon at 2:00pm by a DO2 meter (Model 9150). Water samples were collected at the same place in each pond at 20 cm depth and analyzed for pH (every two days, two times a day, in the morning at 9:00am and in the afternoon at 4:00pm) using a digital pH Meter (Model 410A). Water temperatures was measured three days a week, three times a day at 6:00, 12:00 and 17:00 h at a water depth of 20 cm. It was measured by a thermometer submerged into the pond water and left for 5 minutes, after which the reading was taken with the thermometer still in the water. Water transparency was measured every 2 days at midday using a Secchi disk. BOD was measured every 20 days by the method of Winkler (Andrew et al 1995). COD was measured by the Open Reflux method (Andrew et al 1995).
Abstract: A completely randomized design was used to study growth rate of Tilapia as influenced by pond fertilization (0.133g N/m2/day) with effluents from biodigesters having hydraulic retention time of 10 (ERT10) and 30 (ERT30) days. There were three replications (ponds of 6 m2 in area) of each treatment, applied over a period of 120 days. Growth rate and net fish yield were higher with ERT30 (0.43g/day and 1363 kg /ha) than with ERT10 (0.27g/day and 899 kg/ ha) after 120 days. Mean values for BOD5 were higher for the ERT10 treatment. It is concluded that the improved fish productivity with effluent from biodigesters with 30 day, compared with 10 day, retention times was probably due to a combination of lower BOD in the pond water, and a higher proportion of ammonia-N in the effluent.