When the chips are down at P&P mills

The Donohue Normick pulp and paper mill in Amos, Québec, produces 500 tons of newsprint a day. Photo shows the wastewater treatment facility.

Before a grinder was installed in the treatment plant of a Canadian pulp and paper mill, wood chips and grit in the primary sludge caused the pumping system to clog at regular intervals. As a result, the system required constant monitoring and had to be taken off-line and backwashed at least twice a shift. After a Muffin Monster was installed between the primary clarifier and the mixing tank, the problem has been eliminated.

The Donohue Normick pulp and paper mill in Amos, Québec, is a sprawling complex on the Harricana River. The mill, which uses a thermomechanical pulping (TMP) process, produces 500 tons a day of newsprint, 80-90% of which is exported to the US and overseas.

The mill operates an effluent treatment plant, consisting of primary and secondary treatment, with a capacity of 16,000 m³/day. According to mill engineering supervisor, Jean Fiset, the original problem was created by the pass-through of wood chips from the pulping operation to the clarifiers. Not all the chips in the primary influent would break down during clarification and many would settle to the bottom in the primary sludge. A bar screen was installed to remove the troublesome chips, but not all of the chips were captured.

The resulting problem occurred downstream, in the dewatering operation, where the primary and secondary sludge is combined. The wood chips frequently collected at the inlet of the centrifugal pumps, prior to sludge dewatering, causing the operation to clog and foul. This required taking the pumps off-line and backwashing them to remove the clogged chips. As Fiset explains, "It was a lot of trouble. We were looking at around the clock cleaning and monitoring."

To eliminate this problem a Muffin Monster was installed just before the mixing tank. It now chews the primary sludge, including the wood chips and grit, producing a fine sludge slurry which can be easily and smoothly handled by the pumps. Operating at approximately 200 GPM, the grinder has greatly increased the efficiency of the dewatering operation while saving the mill substantial costs related to pump repairs and system downtime.

Grinders are hard at work at every major pulp and paper company in the US.

This article was abridged from Environmental Science & Engineering magazine, which also contains many more articles not posted on our Web Site. See our home page on how to order your subscription. We regret we can only accept orders from Canada and the United States.