Cryptosporidium removal during water treatment using dissolved air flotation

Abstract

The occurrence of the protozoan parasite Cryptosporidium parvum in water supplies, and the resultant outbreaks of cryptosporidiosis in the UK and USA, have led to concern over the ability of conventional water treatment processes to remove Cryptosporidia from water sources. Large scale pilot plant trials of water treatment have been carried out in the UK to establish the degree of removal that can be achieved by a range of treatment processes, including dissolved air flotation, and to compare the performance of different treatment options. Results from part of these trials are presented in this paper. These results suggest that well operated chemical coagulation based treatment, using either dissolved air flotation or floc blanket clarification, should be capable of achieving removal of Cryptosporidium oocysts of over 99%. There was no evidence of differences in performance between the different types of filter media investigated. The risk of increased Cryptosporidium concentration in the filtered water will increase as filtrate turbidity increases. However, other factors such as high coagulant metal-ion concentration in the filtered water, or a sudden increase in clarified water turbidity, without any increase in filtered water turbidity, may also indicate treatment problems and associated risk from Cryptosporidia. Recycling of backwash waters may also increase the risk.

A review of Cryptosporidium removal by granular media filtration

“…Several pilot-and fullscale studies have demonstrated that organism-sized particles and turbidity are approximate indicators of pathogen removal by drinking water treatment processes but are not reliable quantitative surrogates (Huck et al, 2002(Huck et al, , 2001Emelko, 2001;Nieminski & Ongerth, 1995;LeChevallier & Norton, 1992). In some cases, increased Cryptosporidium passage through filters has been associated with increased filtrate turbidity (Hall et al, 1995;Nieminski & Ongerth, 1995). However, when parameters such as turbidity and particle counts are assessed as surrogates for Cryptosporidium removal by filters, it is important that the surrogates be applicable to the range of conditions encountered during typical operation.…”

“…Despite decreases in oocyst removals relative to stable operation, maintained Cryptosporidium removals of >3 logs during filter ripening have been reported (Swaim et al, 1996). Several pilot-scale studies have indicated that oocyst removals decrease by ~0.5-1 log during filter ripening (Huck et al, 2002(Huck et al, , 2001Emelko, 2001;Swaim et al, 1996;Hall et al, 1995;Patania et al, 1995). These findings were also demonstrated at full-scale by Baudin and Laîné (1998) who reported ~1-log deterioration in oocyst removals during filter ripening.…”

Giardia and Cryptosporidium Removals by clarification and filtration under challenge conditions

“…They found the combination of DAF and dual‐media filtration resulted in Cryptosporidium removals of 4–5 log. Hall et al 13 found DAF and filtration resulted in Cryptosporidium removals of 3–4 log.…”

Physicochemical processes

“…Coagulation and DAF conditions that minimize residual turbidity and maximizes organic removal also produced high log removals of Cryptosporidium. Hall et at. (1995) reported on results from large scale pilot plant tests to determine the degree of Cryptosporidium removal that can be achieved by a variety of treatment processes including DAF.…”

“…Compared to the 30% of dichloroethane lost as air emissions from conventional bioreactors, less than 1.5% of the dichloroethane was lost as an air emission in the membrane bioreactors. Hall et at. (1995) investigated the feasibility of an anaerobic membrane bioreactor for treatment of segregated kraft bleach plant wastewaters and removal of adsorbable organic halogen (AOX).…”