Pulp and paper mill aerated lagoon phosphorus optimization

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The Domtar (formerly Weyerhaeuser) Kamloops, BC bleached Kraft pulp mill discharges treated process wastewater to the Thompson River. The River and downstream Kamloops Lake have historically been identified as nutrient sensitive, with phosphorus contributing to algal growth.

The BC Ministry of the Environment required that the mill perform a phosphorus optimization study to minimize phosphorus discharged in wastewater biologically treated in an aerated lagoon. Untreated mill wastewater is nutrient deficient relative to requirements for effective biological treatment. Supplemental nutrients must be added in a manner that accounts for phosphorus release from settled solids in the aerated lagoon and step changes in influent organic loading associated with different production grades. Aerated lagoon phosphorus behavior was assessed by sampling water quality parameters and microbiology to develop internal profiles from inlet to outlet under differing conditions of influent organic loading and oxygen supply.

A key finding was an oxygen supply-demand imbalance. Phosphorus behavior was strongly correlated with the aerated lagoon dissolved oxygen longitudinal profile and resulting microbiological population. Anoxic conditions in the first and second reaction zones of the aerated lagoon were indicated by both dissolved oxygen and oxidation-reduction potential, along with microbial population characteristics. Anoxic conditions existed despite intensive high speed mechanical surface aeration. Intensity and spatial extent was a function of influent organic load. A pure oxygen addition system was installed to improve the oxygen balance. Orthophosphorus profiles reflected phosphorus release and uptake patterns characteristic of biological phosphorus removal. Changes implemented during the study resulted in more efficient use of added phosphorus per unit of BOD5 removed and lower residual phosphorus discharge.

The Domtar (formerly Weyerhaeuser) Kamloops, British Columbia bleached Kraft pulp mill discharges approximately 30.4 MGD (115,000 m3/day) of treated wastewater. The Thompson River and downstream Kamloops Lake have historically been identified as nutrient sensitive, with phosphorus contributing to algal growth. The British Columbia Ministry of the Environment required that the mill perform a phosphorus optimization study with the objective to minimize phosphorus discharged in treated wastewater. Untreated mill wastewater is nutrient deficient relative to requirements for effective biological treatment. Therefore supplemental nutrients are added. Supplemental phosphorus addition rate control is complicated by variations in release from settled solids in the aerated lagoon and influent organic loading associated with different production grades.

BACKGROUND
The Kamloops pulp mill produces both fully bleached and unbleached market wood pulp from softwood chips and sawdust. Pure oxygen is used in the bleaching process, supplied by on-site generation. Process wastewaters from the pulping, bleaching, and pulp forming operations are sent to an on-site wastewater treatment system. Treatment consists of two stages of primary solids removal followed by aerated lagoon biological treatment. The aerated lagoon is shown in
Figure 1. Figure 2 presents a treatment system schematic.

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