Rethinking the role of in-line coagulation in tertiary membrane filtration of municipal effluents

In-line coagulation with aluminum or iron salts and ultrafiltration (UF) or microfiltration (MF) membranes is a valuable treatment option. The efficiency of the treatment is often evaluated by the achieved separation degree. That separation-oriented approach implies the coagulation with doses that are prohibitively high for many operations including the tertiary effluent treatment. The main purpose of the advanced wastewater treatment however is the retention of microorganisms and suspended solids, and that goal can be achieved even without coagulants. Thus the in-line coagulation can pursue the prevention or minimization of the irreversible fouling as an ultimate goal not related to the maximal separation of organic and inorganic impurities. Pilot experiments at conventional activated sludge (CAS) municipal wastewater treatment plant confirmed that the addition of 1 mg/L Fe³⁺ prevents the irreversible fouling as efficiently as the addition of 5 and 10 mg/L Fe³⁺. The economic impact of the suggested alteration is significant. Estimated operational expenses (OPEX) of a filtration at 60 LMH with 45 min cycles and 1 chemical - enhanced backwash per day is around 2 cents (€)/m³, almost a half of an OPEX of the separation-oriented treatment. Intermittent in-line coagulation down to first 2.5 min of 30 and 45 min filtration cycles is another cost-effective method to successfully depress the fouling. The success is explained by two-stage kinetics of a cake formation. At ripening stage, a layer of flocks restricted by a membrane gradually covers its surface and forms an initial dynamic cake. At operable stage, the cake entraps fresh solutes and prevents their contact with a membrane surface even without a coagulant. A superposition of two approaches reduces the consumption of ferric chloride coagulant by 94%.