Most paper mill lagoons in the US contain earthen basins for water retention and high speed surface aerators to providing for oxygen used by the microbes. Biological solids are removed in polishing ponds which require periodic dredging of sludge to maintain volume.
While the high speed aerators are efficient in adding dissolved oxygen - approximately 2 lb O2/hp/hr. They are less efficient in mixing and result in sludge buildup just outside the zone of influence. As sludge builds, wastewater treatment volume is reduced and short-circuiting of flows becomes an issue. In ares with electric lines and aerators, the mill cannot easily dredge the solids so occasional aerators are moved to help prevent excessive sludge buildup.
While I am not involved with the mill's upgrade proces, I would usually start my optimization process with the following steps.
- Conduct a tracer study for determining hydraulic residence time (HRT) and dispersal/flow patterns in the lagoons
- Survey sludge depth in the ponds using a sludge judge and sonic tests for depth
- Test dissolved oxygen levels, pH, & ORP at locations in the lagoons
- Record N & P levels at key areas of the system
- Move high speed aerators & add mixers to prevent short-circuiting and sludge buildup. This also improves basing mixing to enhance biological treatment by suspending biomass in the high intensity treatment zone
- Evaluated nutrient (N&P) addition to ensure no limits on microbial activity exist due to nutrient limitations
- Evaluate operator control tests to improve knowledge of biological unit "health" and influent impact on biomass
- Develop protocols for use of increased nutrient feed, bioaugmentation (bug) addition based upon control test results
- Dredge pond as needed. Then add low speed mixers to ensure no short-circuiting & proper solids separation. This alone should work to achieve effluent targets.
- In color & odors are still too high, we can look at adding in lagoon treatment units containing immobilized microbes on media.
- Another option is to run part of the effluent through a dissolved air floatation (DAF) unit to remove color. This unit relies on low cost coagulants to increase color particle size to a point where the DAF can remove by skimming the float.