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Treatment and Re-use of Process Water
Dairy and Food Processing Plants
20 September 2005
This presentation was given at the 2005 International Dairy Federation summit in Vancouver, British Columbia, Canada during an Environmental Workshop session.
Please click on the following link to read the Treatment and Re-use of Process Water abstract in text format.
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Why Water Reclamation
The three reasons for water reclamation in a dairy or a food processing plant using RO Polisher:
- Reduce water intake
- Reduce effluent volume
- Produce water free of minerals for process & CIP
Many dairy processors, particularly cheese manufacturers and milk dehydration plants, are net water producers. They take in milk containing about 87 to 88 percent water and produce products such as cheese with perhaps 40 to 50 percent water or dried powders with only 3 to 4 percent water.
Case Study - Plant Description
- Milk Intake: 6 Million lbs /d
- Products:
- 590,000 lbs/d of Cheese
- 25,300 lbs/d of WPC80 powder
- 150,000 lbs/d of Permeate powder
- Plant equipment:
- Milk reception & pasteurizers
- Cheese vats & belts
- Cheese packaging - retail & bulk
- Whey Ultrafiltration systems
- RO preconcentrator for UF Permeate
- MVR evaporator for UF Permeate
- Spray dryer for WPC80
- Spray dryer for permeate powder
- Powder packaging
- CIP system for cheese plant & whey plant
This case study is based on an western United States facility processing 6,000,000 pounds per day of milk into cheese, whey protein concentrate and permeate powders.
Case Study - Water Usage
- Evaporator condensate and RO permeate available for recovery is 690,000 gpd
- 50% of the available water is recovered to produce 310,000 gpd of reclaimed water
- Rest of the water is discharged to the effluent
- Recovered water is used for
- CIP of the whey plant including final rinse
- CIP of the evaporator and spray dryers
- Process water for Ultrafiltration plants
- Boiler feed
- 600,000 gpd of city water used for
- CIP of milk intake
- CIP of Cheese plants
- Potable water for the plant
- Cooling tower makeup
- Hose stations
- Total plant effluent 1.0 million gpd
Approximately half of the available condensate and reverse osmosis permeate stream is recovered and polished using reverse osmosis to yield a high quality water stream that partially replaces the demand for city water.
Reverse Osmosis (RO) Polisher
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Reverse Osmosis membranes retains all suspended and soluble contaminants in water resulting in pure water as permeate. Water from dehydrating equipment such as evaporator or Reverse Osmosis plants can be processed in a Polisher plant to recover the water for reuse.
Reverse osmosis is the pressure-driven, mass transfer of solvent, in this case water, through a semi-permeable membrane by diffusion kinetics.
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The process consists of:
- Collection of the evaporator condensate and reverse osmosis permeate
- pH adjustment to convert trace organic acids into their respective salt forms
- Turbidity monitoring
- Reverse osmosis polishing
- Turbidity monitoring of the recovered water
- Chemical and/or heat treatment
- Storage and distribution
System Operating Parameters
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Recovery
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90% or higher
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Reduction in BOD
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90% or higher
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System Design
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Array
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Operating Pressure
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≤ 30 bar
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Operating Temperature
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20 - 80 C
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Membrane Life
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2 - 4 years
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Reverse osmosis polishing is capable of attaining very high water recovery rates with very low organic contents and is applicable across a wide range of operating pressures and temperatures.
Plant Design
Array Design
- Most Applications
- Most economical
- Highest recovered water quality
The staged-array design offers the optimal solution where initial feed water is high. This represents the most economical solution and the highest quality of recovered water for re-use.
Array Design With Recirculation
- Applications with higher solids loading in feed
- Economical
- Higher recovered water quality
If the initial feed water has a somewhat higher organic content, it is often desirable to internally recirculate at the high concentration end of the plant. This allows for maintenance of the higher cross-flow velocities necessary to ensure good back-diffusion of solutes into the solution bulk-stream.
There is however somewhat of a penalty associated with this approach in terms of economy and recovered water quality.
Recirculation Design
- Applications with high solids loading in feed
- Larger membrane area & higher capital Cost
- Lower recovered water quality
When organic-laden streams are processed, all membrane area is internally recirculated to ensure optimal cross-flow conditions and minimize organic fouling.
This solution however is the most capital intensive and produces a much lower quality of recovered water.
The chart shows a graphical depiction of relative recovered water quality based on operating process design.
Plant Control
- Feed Flow Rate Control
- Concentrate Flow Rate Ratio Control Loop
- Water quality monitoring
Typical control design schemes involve feed flow control with a corresponding cascade ratio control of the reject stream. On-line recovered water quality monitoring is also typical.
Case Study
Capital & Operating Costs for a 300 gpm RO Polisher
- Installed Membrane Area: 1575 m2
- Total capital including installation: $ 350,000
- Total installed HP: 270
- Operating Costs
- Membrane cost based on 2 yr life: $ 30,000/yr
- Labor Costs; $ 10,000 /yr
- Power Costs ($ 0.08 / KwH): $ 85,000 /yr
- CIP Cost: $ 57,000 /yr
- Operating cost for recovering water $ 1.54 / 1000 gallons
- Other costs
- Effluent treatment: $ 1.34 / 1000 gallons
- Effluent BOD treatment: $ 7.98 / 100 lbs
- Effluent Total Suspended solids (TSS) treatment: $ 10.33 / 100 lbs
- Cost of city water in California central valley $ 0.54 / 1000 gallons not including any treatment costs.
This data represents the financial implications of the investment decision for this particular case study.
The relevant comparative costs are $1.54 per thousand gallons of recovered water versus $1.88 for the cost of city water plus the required effluent treatment of un-recovered water, as this would still require treatment but is no longer replacing city water in kind.
Regulatory Issues
FDA Regulatory Requirements
The three (3) general categories for reclaimed water use are:
- Reclaimed water, which may be used for all potable water purposes, including the production of culinary steam.
- Reclaimed water, which may be used for limited purposes, including the production of culinary steam.
- Use of reclaimed water not meeting the requirements of this Section.
RO Polishing of evaporator condensate and/or Reverse Osmosis permeate affords the opportunity of producing a category type 1 water for re-use, the most cost beneficial of the three.
Reclaimed water to be used for potable water purposes, including the production of culinary steam, shall meet the following requirements:
The PMO provides very specific instruction on the criteria for use of recovered water. Highlights revolve around microbiological quality and organic load.
- Water shall comply with the Bacteriological Standards of Appendix G., and, in addition, shall not exceed a total plate count of 500 per milliliter (500/mL).
- Samples shall be collected daily for two (2) weeks following initial approval of the installation and semi-annually thereafter. Provided, that daily tests shall be conducted for one (1) week following any repairs or alteration to the system.
- The organic content shall be less than 12 mg/L as measured by the chemical oxygen demand or permanganate-consumed test; or a standard turbidity of less than five (5) units.
- Automatic fail-safe monitoring devices shall be used to monitor and automatically divert, to the sewer, any water that exceeds the standard.
- The water shall be of satisfactory organoleptic quality and shall have no off-flavors, odors or slime formations.
- The water shall be sampled and tested organoleptically at weekly intervals.
- Approved chemicals, such as chlorine, with a suitable detention period, may be used to suppress the development of bacterial growth and prevent the development of tastes and odors.
- The addition of chemicals shall be by an automatic proportioning device, prior to the water entering the storage tank, to assure satisfactory quality water in the storage tank at all times.
- When chemicals are added, a daily testing program for such added chemicals shall be in effect and such chemicals shall not add substances that will prove deleterious to the use of the water or contribute to product contamination.
- The storage vessel shall be properly constructed of such material that it will not contaminate the water and can be satisfactorily cleaned.
- The distribution system, within a milk plant, for such reclaimed water shall be a separate system with no cross-connections to a municipal or private water system.
- All physical, chemical and microbiological tests shall be conducted in accordance with the latest edition of SMEW.
Allowance for chemical treatment is provided to ensure microbiological as well as organoleptic quality of the recovered water.
Array RO Polishers
Photographs of typical Reverse Osmosis Polisher plants for the recovery and treatment of water from dairy products.
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