Using Chemically Enhanced Primary Treatment (CEPT) as a Pretreatment Option for Anaerobic Digestate from Cattle Manure Digestion System

Revisiting Chemically Enhanced Primary Treatment of Wastewater: A Review

Chemically enhanced primary treatment (CEPT) is a process that uses coagulant and/or flocculant chemicals to remove suspended solids, organic carbon, and nutrients from wastewater. Although it is not a new technology, it has received much attention in recent years due to its increased treatment capacity and related benefits compared to the conventional primary treatment process. CEPT involves both physical and chemical processes. Alum and iron salts are the commonly used coagulants in CEPT. Several types of anionic, cationic, and uncharged polymers are used as flocculants, where poly aluminum chloride (PACL) and polyacrylamide (PAM) are the widely used ones. Some of the coagulants and flocculants used may have inhibitory and/or toxicity effects on downstream treatment and recovery processes. There has been an increasing amount of work on the treatment of wastewaters from various sources using CEPT. These wastewaters can range from municipal/domestic wastewater, combined sewer overflow, landfill leachate, cattle manure digestate to wastewaters from textile industry, pulp and paper mill, slaughterhouse, milk processing plant, tannery and others. In recent cases, CEPT is employed to enhance carbon redirection for recovery and substantially reduce the organic load to secondary treatment processes. CEPTs can remove between 43.1–95.6% of COD, 70.0–99.5% suspended solids, and 40.0–99.3% of phosphate depending on the characteristics of wastewater treated and type of coagulants and/or flocculants used. This article reviews the application, chemicals used so far, removal efficiencies, challenges, and environmental impacts of CEPT.

The impact of chemically enhanced primary treatment on the downstream liquid and solid train processes

The use of chemical coagulants and flocculants to supplement chemically enhanced primary treatment (CEPT) processes is increasing in popularity as it has been demonstrated to improve carbon redirection and suspended solids and phosphorus removal. Dosing 15 mg ferric chloride/L of wastewater and poly aluminum chloride (PACl; 0.5 mg/L) to the influent of a primary clarifier successfully achieved improved carbon redirection and suspended solids removal at a full-scale WWTP. In this study, the impacts of PACl on the downstream liquid and solid train processes of the same WWTP were investigated. Compared to FeCl₃ addition, a combined PACl and FeCl₃ addition to the primary influent reduced the TSS and TP concentrations of the secondary clarifier effluent by 20% and 33%, respectively. Effluent BOD₅ and ammonia-nitrogen concentrations of the downstream activated sludge process were not affected by the addition of a combined FeCl₃ and PACl in the primary clarifier. PACl addition affects the bioavailability of carbon and hence reduced the methane production efficiency of the primary sludge by 20%-30%. However, the significant amount of carbon concentrated in the CEPT sludge would enhance the amount of energy recovered through incineration. PRACTITIONER POINTS: The chemically enhanced primary treatment process is an attractive method for carbon redirection and energy recovery. The combined FeCl₃ and PACl addition in the primary clarifier improves the full scale activated sludge process effluent quality. PACl has a negative effect on methane production.