Filtrate and reject water loops: the hidden internal load

Filtrate and Reject Water Loops: The Hidden Internal Load

Quantifying Your Internal Recirculation Load

Before implementing solutions, facilities must first understand the magnitude of their internal loading. This requires systematic monitoring of filtrate quality from dewatering operations and calculating the mass loading returned to the treatment process.

Key Parameters to Monitor

Focus on measuring total suspended solids (TSS), BOD or COD, total nitrogen (TN), ammonia-nitrogen (NH₃-N), and total phosphorus (TP) in the reject water stream. Flow measurement is equally critical—many facilities underestimate the volume of reject water generated, particularly when operating multiple dewatering units or running extended hours.

Filtrate Quality from Different Dewatering Technologies

The characteristics of reject water vary significantly depending on the dewatering technology employed. Understanding these differences helps in selecting both dewatering equipment and appropriate reject water treatment strategies.

Screw Press Filtrate Characteristics

Screw presses generate filtrate with moderate suspended solids content and relatively lower nutrient concentrations compared to other technologies. The gentle dewatering action minimizes cell lysis, reducing the release of intracellular materials. However, polymer carryover can be significant if dosing is not optimized, potentially impacting downstream biological processes.

Cloth Filter Reject Water

Cloth filters typically produce cleaner filtrate with lower suspended solids, particularly when properly maintained. The filtration mechanism provides effective solids capture, though nutrient concentrations remain elevated. Wash water volumes must be considered in the overall internal load calculation.

Strategies to Reduce Internal Recirculation Load

Breaking the cycle of internal loading requires a systematic approach combining operational optimization, equipment selection, and potentially dedicated reject water treatment.

Optimize Dewatering Performance

Improving cake dryness reduces the volume of filtrate generated per ton of solids processed. For screw presses, this involves optimizing screw speed, back-pressure settings, and polymer dosing. Cloth filters benefit from proper tension adjustment, regular cleaning cycles, and appropriate polymer selection.

Flow Equalization and Strategic Timing

Rather than continuous return of reject water, implement equalization storage and return filtrate during periods of lower influent loading or higher biological treatment capacity. This prevents shock loading and allows the biology to process internal loads more effectively.

Dedicated Reject Water Treatment

For facilities with severe internal loading issues, dedicated treatment of reject water before return to the main process may be justified. Options include physical-chemical treatment, biological side-stream processes, or advanced oxidation.

Reject Water Treatment Options

When operational optimization alone cannot sufficiently reduce internal loading, dedicated treatment becomes necessary.

Physical-Chemical Treatment

Coagulation and flocculation can remove suspended solids and phosphorus from reject water before return to the main process. This approach is relatively simple to implement but generates additional sludge and chemical costs.

Biological Side-Stream Treatment

Technologies such as anammox, sequencing batch reactors, or moving bed biofilm reactors can treat high-strength reject water in a dedicated system optimized for these conditions. While capital-intensive, these solutions can dramatically reduce nitrogen loading to the main biological treatment process.

Membrane Filtration

Ultrafiltration or microfiltration can polish reject water, removing suspended solids and reducing organic loading. This approach works well in combination with biological treatment for comprehensive reject water management.