Secondary sludge thickening, stabilisation and disposal

6. Secondary sludge thickening, stabilisation and disposal

Although the backwash water flow from the microscreens may be less than 0.1 of the primary flow, this still requires dewatering. Sedimentation is well suited for this purpose because the flow rates are markedly reduced Cripps and Kelly, 1996; Summerfelt, in press. There are several potential ways for beneficial disposal of organic waste from aquaculture: application on agriculture land, composting, vermiculture and reed drying beds Tchobanoglous and Burton, 1991; Summerfelt, in press. Newly produced sludge from aquaculture is considered a good ‘slow-re- lease’ fertiliser in agriculture with a high concentration of organic matter, nitrogen and phosphorus, but with a low potassium content Bergheim et al., 1993a,b; Westerman et al., 1993. Test application rates of such sludge for plant production have been reported Willett and Jakobsen, 1986; Myhr, 1989. Composting has been reported as suitable for treating fish farm sludge Bergheim et al., 1998. A system combining effluent treatment and sludge processing by sedimentation and lime stabilisation is presented in Fig. 3. In tests, the sedimentation unit of this commercially developed system removed 85 – 90 of the solids at an overflow rate of about 1 m h − 1 , producing a sludge with a dry matter content of 5 – 10 after 24 h settling Bergheim et al., 1998. Throughout the whole process, from primary flow to stabilised sludge, the solids content increased by a factor of about 20 000. This system produced on average 0.7 l of settled sludge 10 dry matter per kg feed supplied at a FCR of about 1 kg feed kg − 1 gain. In order to achieve a stabilised sludge pH \ 12, a lime dosage of about 15 g CaO per l of sludge was required. After storage for up to 3 months the sludge should then be suitable for land application as an agricultural fertiliser. Trace metal concentrations in the sludge may need to be monitored to ensure that they are below regulatory limits. Fig. 3. Diagram of an integrated effluent treatment and sludge processing system developed in conjunction with Sterner Aquatech a.s., Sweden. The capital and running costs for effluent treatment that are considered accept- able are dependent on the size of the farm and the potential profit margin Muir, 1982. In a review of some effluent treatment systems used in European salmonid production, Bergheim et al. 1998 found that the treatment costs amounted to 2 – 10 of the total production costs.

7. Conclusions