Effects of recirculating water treatment sludge

Abstract: Drinking water is crucial in our everyday life. As with many other processes, it is important to produce water of drinking quality in a sustainable manner. This can occur by e.g., decreasing the amount of chemicals in the process. When producing drinking water, coagulation and flocculation are common processes. Briefly, coagulation is the process when negatively charged organic matter is destabilised by coagulants and during flocculation, the destabilised organic matter aggregates into flocs. These flocs settle and can be removed from the treated water. Settled flocs are called sludge and contains organic matter and residuals from coagulants, which are typically iron or aluminium salts. This thesis was carried out in collaboration with Sydvatten AB, a drinking water producer for western Skåne and the experiments were performed at Ringsjöverket, one of the drinking water plants which Sydvatten owns. The main objective was to investigate the effects of recirculating sludge in the treatment process and if it was possible to decrease the amount of coagulant when recirculating sludge. The laboratory work was conducted by using jar tests which are common for evaluation of coagulation and flocculation. The absorbance of the treated water was measured, and the lower measured absorbance the more organic matter had been removed. The used wavelength were 254 nm and 436 nm. Similar to the large-scale process, water from lake Bolmen in Småland was used. However, instead of using ferric chloride, a polyaluminium chloride coagulant (PAX-XL60) was used. When recirculating sludge, absorbance decreased for low coagulant dosages (3–5 mg Al/l), and for higher dosages (6–8 mg Al/l), the absorbance remained unchanged. It was therefore concluded that recirculating sludge had the greatest effect on the lower coagulant dosages. Additionally, the absorbance decreased significantly for coagulant dosage 4 mg Al/l with sludge compared to the same dosage without sludge. Furthermore, recirculating sludge with 4 mg Al/l gave absorbance results comparable to 5 mg Al/l without sludge. It was concluded that when recirculating sludge, coagulant dosage decreased with 20 % and the degree of recirculation was 23 %. The amount of produced sludge can decrease as a result from using less coagulant which is beneficial from a sustainable perspective. Furthermore, operational costs will decrease when using less coagulant. Before the recirculation takes place in the large-scale process, further experiments are recommended. One of these is investigating how the microbial barrier in the coagulation and flocculation processes is affected when recirculating sludge. It is also recommended to examine how the temperature varies throughout an entire year and its effect on the flocculation process.