Dewatering technology for wood pulp effluent

This is based on an actual case study involving effluent from a chemi-thermo-mechanical pulping process. The plant was not meeting effluent discharge standards due to poor removal of wood fibre solids and BOD/COD from their effluent and were also spending tens of thousands of dollars per month on polyelectrolyte dosing ahead of the solid-liquid separation system. As a result they were facing expenditure of many millions of dollars for a biological plant upgrade.

Equipment used prior to the Flowmax: Two rotating wedge-wire screens were used for solids removal, followed by a belt press to further dry the solids. The filtrate from the screens went to a dissolved air flotation (DAF) unit where additional solids were removed.

Flowmax trial basis: A 12m long Flowmax 4400 was utilised for the trial, with the full effluent stream from the plant (an average flow of 180 m3/h) being pumped from a sump into the Flowmax. Sludge returning from the biological treatment plant was also pumped into the Flowmax at the same time. Filtrate was piped to the DAF plant feed channel.

Solids from the exit conveyor were fed into the Rampress, the expectation being that if a solids concentration over 40% could be produced and the wood fibre solids compressed into a cylinder, the resulting wood cylinders could be dropped directly on to the grate of a co-generation plant on the site as fuel.

Results: The Flowmax 4400 delivered total suspended solids (TSS) reduction from 11,142ppm down to 181ppm, using only 0.3kg/dry tonne PEO (98.4% reduction). They were previously achieving 97.5% using rotary screens and a DAF, but at tens of thousands of dollars more per month in polymer dosing. The solids came off the Flowmax at up to 30% concentration and, after the ram press, 45% as a compact cylinder which can be fed to the co-generation plant on site as fuel.

The Technical Manager at the plant had this to say about the performance of the Flowmax 4400:

Separation performance by the Flowmax 4400 exceeded all expectations, achieving a 98.4 % reduction with a small amount of polymer.

By significantly increasing the amount of BOD removed from the effluent stream the Flowmax delivered a result which could potentially save millions in upgrade costs to the biological treatment plant.

A flow rate of 180m3/h was achieved with a very fine aperture belt, enabling the full effluent stream to be treated with one machine.

Dried solids from the following ram press achieved over 40% solids concentration in solid cylinder form able to be utilised by the co-generation plant as a fuel source.