Practical and cost-effective solutions for the removal of dissolved phosphorus from fish farm effluents are lacking. C alcium-rich crushed concrete has proven to be effective at removing phosphates from other types of wastewater, and this study therefore examined if crushed concrete can be effective at treating aquaculture wastewater, too. The phosphate sorption capacity of crushed concrete was investigated in laboratory batch experiments as well as in flow-through filters in the laboratory and in field experiments.
The laboratory batch experiments showed that crushed concrete achieved phosphate removal capacities above 200 g PO4 -P/kg concrete due to calcium phosphate precipitation at high pH in addition to P binding to concrete. Furthermore, the studies showed that the P sorption capacities of crushed concrete were highly dependent on P concentrations in the water as described by Langmuir adsorption isotherms.
The laboratory flow-through column tests showed that crushed concrete filters operated at different hydraulic retention times (HRT) removed phosphates at rates of up to ~9 g P/m3 /d at an HRT of approximately 4 hours . Furthermore, the tests showed that h igh effluent pH values from the concrete were effectively neutralized by downstream woodchip bioreactors.
The field experiments were carried out at a recirculated trout farm in Denmark and showed that crushed concrete filters run over 20 weeks were able to remove up to 3 g P/m3 /d at an HRT of 7h without showing signs of filter clogging.