Reactive filter materials for ammonium and phosphorus sorption in small scale wastewater treatment

Abstract: Existing small scale wastewater treatment systems in Nordic countries have generally low treatment efficiency regarding phosphorus and nitrogen. Even though only 15% of the Swedish population is connected to on-site wastewater treatment systems, the total discharge of phosphorus from these systems exceeds the total discharge from all municipal treatment plants in Sweden. The overall objective of this thesis was to investigate certain components of small wastewater treatment systems that improve the effluent quality and facilitate the recovery of wastewater phosphorus and nitrogen. Reactive filter materials intended for filter systems and local sludge handling were investigated. The experimental methodology used to investigate reactive filter materials was also examined. The materials studied were blast furnace (BF) slag, wollastonite and clinoptilolite, which were investigated by agitation, column and pilot scale experiments as well as by literature studies. Ammonium and phosphate solutions and municipal wastewater were used in the experiments. The local sludge handling was investigated by a full-scale field study consisting of sludge freezing, drying and composting. Factors influencing phosphorus (P) sorption on BF slag in agitation experiments were grain size, weathering, initial P concentration and type of solution. The highest P sorption value obtained was 1,500 mg tot-P/kg slag for fresh BF slag treating a PO4-P solution of 20 mg tot-P/L. The results showed that the properties of BF slag had changed during storage, and the handling of BF slag before being used in a reactive filter system may affect the P treatment efficiency of full-scale filters. Considerable release of sulphuric compounds was seen in the pilot scale experiment even though the used slag was weathered. Thus, the use of fresh slag to optimise the P sorption may be a problem when considering sulphuric leakage. The P sorption of wollastonite was 90-93% for initial concentrations 14-61 mg PO4- P/L and the sorption was 190 mg PO4-P/kg wollastonite for a solution of 14 mg PO4-P/L. For the investigated wastewater, the PO4-P sorption of wollastonite was negligible. Wollastonite showed minor NH4+ removal, varying between 3 and 15%. Neither the reaction times nor the initial ammonium and phosphate concentrations influenced the ammonium sorption. In column experiments, where clinoptilolite was loaded with wastewater, the highest adsorption was obtained for the finest (4-8 mm) clinoptilolite, i.e. 2,700 mg NH4-N/kg. Breakthrough of ammonium occurred immediately in all clinoptilolite experiments. Laboratory experiments are easily performed with prepared solutions instead of wastewater. However, obtained sorption values from prepared solutions and wastewater differ, as shown in agitation experiments where the P sorption for fresh BF slag was almost 100% for an initial phosphate solution of 5 mg tot-P/L. The corresponding P sorption for a wastewater solution with an initial concentration of 4.2 mg tot-P/L was 56%. Filter experiments using wastewater are the laboratory experiment that can best forecast the performance of full-scale applications. A sorption capacity of the filter material can be obtained as well as information about other operation parameters, such as suitable grain size, hydraulic load and risk of clogging. Most small scale wastewater treatment systems include a sludge separation unit that separates a large fraction of the wastewater's suspended solids. The investigated full-scale septic sludge application was a promising method for septic sludge treatment in cold regions, due to its simple design and operation. Freezing and drying efficiently dewatered the sludge. The dried sludge and kitchen refuse were then intensively composted and a maximum temperature of 73 degrees C was achieved. Using the investigated sludge treatment method would contribute to the local recovery of wastewater nitrogen and phosphorus from small-scale wastewater treatment systems by approximately 5-20%.