Should flocculants be added prior to biochemical treatment? Three core indicators to help you make an accurate assessment.

In wastewater treatment processes, pre-treatment prior to biological treatment is a critical step in ensuring the stable and efficient operation of subsequent biological systems. The decision to add flocculants at this stage should not be based on experience alone; the core lies in the scientific analysis of the influent water quality. By mastering the following three key assessment indicators and combining them with pilot-scale testing, precise decisions can be made while selecting the appropriate flocculant type. I. Three Indispensable Key Determinants 1. Determinant One: Settling Performance of Suspended Solids (Verified via Static Settling Test) This constitutes the most intuitive and readily implementable on-site assessment method, providing rapid insight into the stability of suspended solids (SS) within the water. Take an appropriate volume of raw water and place it in a graduated cylinder. After allowing it to settle for 30 minutes, observe the following: No addition required: If sludge settles rapidly and the upper layer becomes clear and transparent, this indicates easily settable suspended solids that will not impose excessive burden on subsequent biological treatment systems. Flocculant addition is typically unnecessary. Requires addition: If the sample remains turbid after settling, with slow sedimentation of suspended solids or only the formation of fine flocs that struggle to settle, this indicates the presence of substantial colloidal particles or minute suspended impurities. Such substances possess high stability and will impede subsequent biochemical reactions. Flocculants must be added to disrupt their stability, causing them to coagulate into larger flocs for efficient sedimentation and separation. 2. Indicator Two: Analysis of Core Water Quality Parameters By monitoring key water quality indicators, pre-treatment requirements can be quantitatively assessed, avoiding indiscriminate chemical dosing: Suspended Solids (SS) and Turbidity: When influent SS concentrations exceed 150–200 mg/L, or turbidity is markedly elevated, substantial suspended solids may adhere to microbial surfaces, clog biofilms, increase biochemical unit loads, or even trigger sludge bulking. Pre-treatment with flocculants to remove partial SS can alleviate the biochemical system’s burden. COD Composition: Separately analyse total COD and dissolved COD. If total COD substantially exceeds dissolved COD, this indicates substantial suspended and colloidal COD in the water. Such COD is difficult for microorganisms to utilise directly. Adding flocculants for prior removal reduces the organic load on the biological system and lowers operational energy consumption. Total Phosphorus (TP): Should the influent TP concentration be elevated, particularly with high inorganic orthophosphate levels, flocculants (e.g., aluminium or iron salts) must be applied for chemical phosphorus removal. Direct entry into the biological system would render microbial phosphorus removal insufficient for compliance, likely resulting in final effluent phosphorus exceedances. 3. Indicator Three: Assessment of Biochemical System Adaptability Even if the aforementioned indicators do not exceed the ‘exceedance’ threshold, the pre-addition of flocculants is recommended under the following circumstances: The influent contains substantial quantities of colourants and colloidal substances that are difficult to biodegrade. Such substances impair microbial nutrient uptake and may inhibit microbial activity; Frequent fluctuations in influent quality (e.g., intermittent discharge of industrial effluent). Pre-treatment with flocculants can form a ‘buffer layer’ to stabilise influent quality, safeguard microbial community stability within the bioreactor, and prevent system collapse due to water quality shocks. II. Flocculant Selection Guide Based on the aforementioned assessment results, selecting the appropriate flocculant according to water quality characteristics is essential to achieve optimal pretreatment outcomes: 1. Inorganic Flocculants (Coagulants): Primarily used for colloid destabilisation These agents destabilise colloidal particles by compressing the double electric layer and neutralising colloidal negative charges. They are suitable for wastewater dominated by colloids, fine suspended solids, and inorganic orthophosphates: Poly Aluminium Chloride (PAC): Widest application, suitable across broad pH range (5.0–9.0), rapid floc formation, excellent phosphorus removal and suspended COD reduction, high cost-effectiveness; Polyferric sulphate (PFS): Forms dense flocs with rapid settling rates, exhibits tolerance to low temperatures and high turbidity, making it suitable for treating low-temperature, low-turbidity water. Residual iron ions have minimal impact on effluent colour. 2. Organic polymer flocculants (coagulation aids): Primarily employed to enhance flocculation Typically used in conjunction with inorganic flocculants, these agents promote the aggregation of destabilised microflocs into coarse, compact flocs through adsorption bridging and entrapment mechanisms, thereby improving sedimentation efficiency: Anionic polyacrylamide (APAM): Suitable for treating high-concentration, positively charged inorganic suspensions (e.g., silt, tailings wastewater). Must be applied after inorganic flocculants to significantly enhance floc strength; Cationic / Non-ionic Polyacrylamide (CPAM / Non-ionic PAM): More suitable for wastewater with high organic content (e.g., food processing, textile dyeing effluents), effectively adsorbing organic colloids; CPAM also finds extensive application in subsequent sludge dewatering processes. Summary and Core Recommendations The decision to add flocculants prior to biological treatment hinges on a comprehensive assessment of three key indicators: ‘settling properties of suspended solids, COD composition, and total phosphorus concentration’. The key to determining the optimal chemical type and dosage lies in conducting ‘cup tests’: these simulate actual plant conditions to observe floc formation rate, floc size, and supernatant clarity under different chemicals and dosages, thereby establishing a precise dosing regimen. This approach ensures effective pretreatment, reduces the burden on the biological system, avoids chemical wastage, and achieves the optimal balance between pretreatment efficacy and operational costs. The main products of flocculant manufacturer Xinhuan Water Treatment are: White polyaluminium chloride (food grade), drinking grade polyaluminium chloride, industrial grade polyaluminium chloride, anionic polyacrylamide, cationic polyacrylamide, nonionic polyacrylamide, petroleum recycling polyacrylamide, oil recovery polyacrylamide, polyacrylamide, polymer flocculant polymer flocculation coagulant, polymeric ferrous sulphate, polymerized iron and aluminium chloride, and other water treatment chemicals. Welcome customers to write to us to negotiate business! Tel/WhatsApp：86 19139972558 Email：Sunny@xhwtm.com Contact:Sunny