By Jason C. Boyd
One of the critical issues in papermaking is the demand for uninterrupted fresh water. Many manufacturers are turning to water re-use, essentially converting waste streams into alternative water sources. One paper mill in Arizona evaluated the application and integration of membrane technology to facilitate water reuse. With an imminent expansion designed to double the plant’s production, the mill needed a cost effective way to deal with discharge and water supply. Water Reuse seemed like a good fit.
|An integrated MBR/RO pilot unit was used to treat the mill’s waste stream.|
At the time, the mill utilized a cloth disc filter to reduce total suspended solids prior to discharge to the wastewater treatment plant. Treatment challenges included polymer, wet strength resin, dry strength resin, and papermaking chemicals. The water treatment and supply system needed to be revamped. In order to reuse its wastewater, the mill would need a system that could meet the quality requirements of the papermaking process. A poor water treatment selection could result in productivity and quality issues that are known to arise from the accumulation of dissolved and colloidal substances. A membrane bioreactor (MBR) was considered to help achieve the mill’s goals of increased water supply and waste reduction.
Beginning in May 2011 and ending in July 2011, a pilot study was conducted by Layne Water Technologies to determine the feasibility of taking currently discharged water and treating it to quality standards suitable for reuse within the paper plant. The 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 paper making, and that the proposed configuration of treatment equipment is suitable for this industrial application.
An integrated MBR/RO pilot unit was used to treat the mill’s waste stream. The wastewater discharge was sent to a four gpm MBR pilot consisting of a biological tank and membrane tank. The effluent from the MBR was sent to a four inch single-module reverse osmosis filter for de-mineralization. The biological system prior to the membranes consisted of coarse bubble diffusers. Positioned after the MBR, an RO filter removed nitrates, biological phosphorus and chemical phosphorus.
Data was collected for flow, flux, removal rates, dissolved oxygen (DO) in the reactor, transmembrane pressure (TMP), temperature and mixed liquor suspended solids (MLSS). Sampling frequency varied depending upon the parameter but included daily, weekly and bi-monthly sampling.
During the study the MBR operated at high flux rates ranging from 18 to 30 GFD. The product water from the MBR had a 99.3% BOD reduction, 87.8% COD reduction and 100% TSS removal at all flux rates. There were no signs of fiber breakage or fouling on the MBR and RO during the pilot.
Operating Cost Reduction
Based on the MBR/RO pilot performance data and plant’s current operating practices, Layne has identified the following operating costs that are positively impacted through the proposed MBR/RO improvement:
Currently, the average felt life on a papermaking machine is about 4-5 weeks and the felts are changed approximately 11-12 times per year. Optimum felt and press section runability depends on maintaining “like-new” characteristics for a longer period of time. Generally, conditioning water usage per felt should be 5-8 gpm/1000 running square feet of felt. An MBR would allow for the use of more water on the flooded nip and other cleaning showers which could add at least a week to the felt life. Increasing the felt life by one week saves 2-3 felts per year which could save between $45,000 and $60,000.
The machine is shut down for 6-8 hours every 4-5 weeks to change the felt. By increasing the felt life the preventative maintenance shut down can be delayed another week to every 5-6 weeks, which will eliminate three shut downs per year. Each hour of shut down can cost anywhere between $2,000 to $3,000.
MBR effluent can be used for seal water, which has a flow rate of approximately 50 gpm. Currently this is fresh water and all of it goes to the drain.
|A simple process flow diagram of the pilot unit.|
MBR product water is a good source for RO feed water and having a consistent, good quality feed water will significantly increase the RO performance and efficiency. Also, this reduces the consumption of antiscalant, biocide, and other chemicals. Improved RO feed water can increase the water recovery rate from 70% to 80%.
Pumping of groundwater can be decreased significantly and will close the loop by 75-80%. The water will always be of good quality and will meet the fresh water quality standards.
The cost for wastewater discharge is approximately $4,500 dollars for 200,000 gpd. If the flow is doubled through the addition of a second papermaking machine the cost of wastewater discharge could be approximately $10,000. Using MBR treatment, this could be reduced significantly or completely eliminated through water re-use.
Based on previously obtained data, the following power consumption savings of the centrifugal pumps have been calculated:
Estimated pump operating costs:
-- SV33 @ 25 HP (5 HP per stage) for two pumps $27,515 per year.
-- SV33 @ 20 HP (Equivalent of 4 stages) for two pumps $22,012 per year.
Savings of $5503.
From a third party water analysis and the data collected by the pilot, a predicted cost for the Antiscalant was determined. By blending MBR filtrate with 20% well water (due to the lower concentration of iron and other impurities) the required Antiscalant dosage can be greatly reduced.
Estimated Antiscalant costs for two RO Systems:
-- 8 ppm of antiscalant is 1.45 gpd per RO system. Total cost for both systems is $24,057* per year.
-- 3 ppm of antiscalant is .54 gpd per RO system. Total cost for both systems is $8,959* per year. (*costs of chemicals may vary from vendor to vendor, based on cost of $1250/ 55 gallon drum.)
Savings of $15,098 + bleach or bisulfite savings
The paper mill can capitalize on significant savings by using the MBR process as RO feed and reusing water throughout the plant and system. Reuse will allow it to be less reliant on the municipality and to control their water and wastewater program.
Results proved that the MBR/RO is a significant improvement over the current system and that membrane technology enables the client to exploit its own waste stream as an alternate source of fresh water. The consistent water quality from the MBR & RO can help increase production while decreasing water and wastewater expenses, reducing environmental impact and reducing client operating costs. The product water from the RO meets the water quality requirements for papermaking and is suitable as an alternative process water source.
Author: Jason C. Boyd, National Marketing Manager – Wastewater, Layne Christensen Company, Water Technologies Group. For more information, visit www.laynewater.com.