A two-month pilot study in the water-intensive pulp and paper industry demonstrated that a mill’s wastewater stream could be turned into an essential resource for increasing its production and reducing costs.
While freshwater is increasingly recognized as a diminishing resource, a critical issue in the papermaking industry is the demand for an uninterrupted supply of freshwater. A mill can consume as much as 15,850 gal for every ton of paper produced.
Seeking to increase its production without incurring further wastewater treatment costs, a mill in the southwest U.S. began looking for alternatives to its practice of sending its wastewater to the municipal water utility.
Layne Christensen Co. conducted a pilot study for two months using Layne’s Poreflon membrane bioreactor (MBR) followed by RO, directly treating the wastewater from the mill.
“Objectives of the pilot study were to demonstrate that water reuse is financially beneficial for the paper mill, that the plant’s waste stream can be successfully treated to meet the quality requirements of papermaking, and that the proposed configuration of treatment equipment is suitable for this industrial application,” said Vetrivel Dhagumudi, Layne Christensen application engineer.
The challenge arose within the various solutes and traces of chemicals in the wastewater itself, resulting from the diversity of products, raw materials and different production processes at the mill. Polymer, wet strength, dry strength and papermaking chemicals, among others, were common in the waste stream.
Special attention also was needed to ensure that the reuse water quality met the requirements for paper manufacturing.
The wastewater discharge was sent to a 4-gal-per-min (gpm) MBR pilot consisting of a biological tank and a membrane tank. The effluent from the MBR was sent to a 4-in., single-module reverse osmosis pilot unit for further treatment to de-mineralize the water to meet papermaking quality.
During the study, the MBR was operated at various flux rates ranging from 19 to 30 gal per sq ft of membrane per day.
Although sporadic behavior was observed in the effluent from the mill, results demonstrated superior water quality for the papermaking process. This sporadic behavior was reflected by unpredictable biochemical oxygen demand (BOD)/chemical oxygen demand (COD) loading in MBR influent.
In three consecutive days, observed COD values were from 330 mg/L to 890 mg/L to 371 mg/L. Observed BOD values measured from 500 mg/L to 60 mg/L in one day and total suspended solids (TSS) values measured from 500 mg/L to 140 mg/L in just a few days.
However, the Poreflon MBR treated to the highest quality possible throughout the changes in loads. An average COD removal of 87.8% was achieved with an average COD concentration of 55 mg/L in MBR effluent.
Even at high loading rates, no BOD was detected in MBR effluent during the entire study, and almost 100% BOD removal was achieved. Lab test results also showed that TSS and ammonia were both undetectable for every sample during the entire study as well.
Thus, Layne’s MBR/RO technology enables the mill to exploit its waste stream as an alternate source of freshwater while reducing discharge fees paid to the local municipal utility.
“The consistent water quality from the Poreflon MBR, and RO can help increase production while decreasing water and wastewater expenses, reducing environmental impact and reducing client operating costs,” Dhagumudi said. “The RO product water meets water quality requirements for papermaking and is suitable as an alternate process water source.”
Vetrivel Dhagumudi is an application engineer at Layne Christensen Co. Dhagumudi can be reached at [email protected]. Dr. Dongxu Yan is an application engineer at Layne Christensen Co. Yan can be reached at [email protected]. Matt Bokor is a consultant for Layne Christensen Co. Bokor can be reached at [email protected].
