Static mixer continuous chemical coagulation–flocculation for cattle feedlot wastewater treatment

Feedlots generate a large amount of manure-contaminated runoff wastewater that is accumulated in reservoirs and should be treated promptly before disposal. A promising alternative to reduce the organic level while recovering the nutrients in a timely manner is a physicochemical treatment based on coagulation–flocculation. This work aims at optimizing the coagulation–flocculation stage of a feedlot wastewater treatment. First, batch experiments were carried out to define and optimize the dose of coagulants. Then, continuous coagulation was examined inside a horizontal tube, empty and provided with static mixers. Iron chloride, calcium hydroxide and magnesium hydroxide were tested as coagulants. The best strategy found was a combined addition of iron chloride followed by calcium hydroxide. Removal efficiencies of around 98% (phosphorus and organic matter) were achieved with a dose of 10 mM iron (III) plus the calcium hydroxide required for neutralization. The use of a Koflo type static mixer was found to be the best configuration, promoting the formation of flocs with excellent settling properties. Settler areas up to eight times smaller would be required if the flocculation was promoted in the static mixer rather than in a stirred tank. The proposed method would be convenient for both small and large feedlot establishments to self-treat wastewater shortly after a rainfall.