Paper mills are notorious for holding solids too long in the clarifier

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Paper mills are notorious for holding solids too long in the system.

Why does it matter how long you hold the solids?

Holding papermill influent or effluent and associated solids too long generally causes not only septic conditions, which can generates odors, but also generates low D.O. conditions. Low D.O. conditions cause the bacteria to generate specific compounds with sulfides and/or organic acids.

Holding times of influent should always be kept to a minimum. Generation of septic conditions can also contribute to ashing in the clarifier, since the solids are turning anaerobic in the bottom of the clarifier and generate gases that cause clumps of solids to float to the top. This increases solids carryover. These conditions usually lead to the growth of excessive levels of filaments later on in the aerobic biological portion of the system. Sometimes, anaerobic sludge is sent to the primary clarifier via the belt press supernatant. This can also lead to an increase in septic conditions in a primary clarifier, since you are seeding the system with bacteria that like to grow in anaerobic or facultative conditions.

Tissue mill in Wisconsin, Algae and scum clogging up weirs

Text Box: Tissue mill in Wisconsin, Algae and scum clogging up weirs 100% recycle mill- floating solids on clarifier

Floating islands due to too many solids

Scum build-up in the primary clarifier

Algae build-up in secondary clarifier can cause permit violations on TSS and false BOD readings.

Ashing on clarifier - and Gassing due to holding solids too long, and the bacteria turning anaerobic and generating gasses and causing solids to float

Some papermills have activated sludge Some Have SBR's or Anaerobic digestors

95% of all papermills that we have seen that have SBR- Sequencing Batch Reactors all have filamentous bacteria problems. One flaw in the fill and draw sequencing is that it very rarely is altered by the operators. A preset time for each segment of fill, aerate, settle and draw down is designated at the time of design and installation. Unfortunately, someone in the front end of the papermill was not informed, or flow rates change, production increases or decreases, etc. Either way, the settle time often is too long or solids build-up in the tank is too long. The bacteria in the tank run out of air during the decant phase and float solids up to the top of the tank as seen on the right picture. Left is the same tank during aeration. Changes were made to the schedule at the plant to alleviate scum, carryover and filamentous growth.

Some have equalization tanks. These are notorious for again holding solids too long, low or no D.O., generation of organics acids that lead to filamentous problems.

Lagoons are another source of solids build-up. This is a solids holding lagoon. Run-off causes problems, since leachate returns to the front of the system.

Some Papermills have large lagoons. Below is an example of a three stage lagoon.

The first stage of the lagoon was aerated, second and third settling lagoons. Large amounts of algae, scum and duckweed covered the last two stages of the pond. Many lagoons have large areas for settling. Duckweed can often grow in places where BOD removal is not completed.