Nitrogen and phosphorus for pulp and paper mills must be available in sufficient amounts to support bacterial growth during biodegradable organics removal. In most pulp and paper mills, supplemental nitrogen and phosphorus must be added to ensure an adequate supply for bacterial growth. But nutrient enrichment of receiving waters is a major water quality issue in North America. The USEPA, states, and provinces have, or are adding, nitrogen and phosphorus discharge limits to pulp and paper mill wastewater discharge permits; sometimes at very low levels.
Many mills already have limits for one or both nutrients and are struggling to achieve compliance, both using aerated lagoons and activated sludge processes. Targeted aeration and mixing is important in both cases. Aerated lagoons have a specific problem because biological solids, by design, are allowed to settle in the lagoon, especially near the discharge end. Settled solids form an anaerobic sludge layer where the solids decay, releasing nitrogen in the form of ammonia and phosphorus in the form of orthophosphate. This benthal release of nitrogen and phosphorus near the final discharge results in elevated concentrations that can endanger compliance with permit nutrient limits.
The problem can be lessened by careful design and operation of aeration and mixing, controlling biological solids settling in a manner that maximizes internal reuse of the released ammonia and orthophosphate, and minimizes the amount of supplemental nutrients that must be added for effective biological treatment. Contact a Pulp & Paper expert today to discuss a customized solution for your wastewater treatment needs.
Pulp & Paper Links: