Treatment and Re-use of Process Water

2005 IDF World Dairy Summit
Treatment and Re-use of Process Water
Dairy and Food Processing Plants
20 September 2005

Introduction

Many dairy processors, particularly cheese manufacturers and milk dehydration plants, are net water producers. They take in milk containing about 87 to 88 per-cent water then produce and sell products such as cheese with perhaps 40 to 50 percent water content or dried powders with only 3 to 4 percent water. Therefore, a byproduct of their production processes is water.

Couple this with an increasing scarcity of good quality water supply to support these manufacturing processes and stricter regulatory constraints on waste water discharge from these facilities and you can begin to get a sense of the drivers toward water recovery and re-use in these types of facilities.

One approach that is being successfully applied to this problem is the use of Re-verse Osmosis to treat various recovered process water streams to produce an excellent quality of water for re-use within the process plant. The three main rea-sons for water reclamation and treatment in a dairy or a food processing plant are:

Reverse osmosis is the pressure driven mass transfer of solvent, in this case wa-ter, through a semi-permeable membrane by diffusion kinetics. Reverse Osmosis membranes retain all suspended and soluble contaminants in water resulting in pure water as permeate. Water from dehydrating equipment such as evapora-tors, generally referred to as condensate, or Reverse Osmosis plants, generally referred to as permeate, can be processed in a Polisher plant to recover the wa-ter for reuse.

Case Study

A case study is offered to further explain the process and demonstrate the eco-nomics. It is based on a western United States facility processing 6,000,000 pounds per day of milk into cheese and whey protein concentrate and UF per-meate powders.

This facility is generally described as follows:

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.

A summary of water resources, recovery, discharges and uses is as follows:

• Evaporator condensate and RO permeate available for recovery is 690,000 gallons per day (gpd)
• 50% of this available water source is recovered to produce 310,000 gpd of reclaimed water
• The remainder 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

The process water recovery and treatment consists of:

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.

Some key operating data is presented below:

Reverse osmosis plant design and construction can vary considerable depend-ing on water source and quality, re-use objectives and specifications, and operat-ing conditions.

The staged-array design offers the optimal solution where initial feed water qual-ity is relatively high. This design represents the most economical solution and the highest quality of recovered water for re-use.

If the initial feed water has a somewhat higher organic content, it is often desir-able to internally recirculate at the high concentration end of the reverse osmosis 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 econ-omy and recovered water quality.

When organic-laden streams are processed, all membrane area is internally re-circulated to ensure optimal cross-flow conditions and to minimize organic foul-ing. This solution however is the most capital intensive and produces a much lower quality of recovered water.

Typical control design schemes involve feed flow control with a corresponding cascade ratio control of the reject stream. On-line recovered water quality moni-toring is also typical. These integrated control approaches can insure the proper balance between water recovery rate and quality.

The following data represents the key capital investment and operating cost data for this particular case study. Monetary values are expressed in U.S. dollars.

• 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 in-cluding any treatment costs.

The relevant comparative costs are $1.54 per thousand gallons of recovered wa-ter versus $1.88 for the cost of city water plus the required effluent treatment of un-recovered water ($0.54 + $1.34), as this would still require treatment but is no longer replacing city water in kind.

Regulatory considerations

Dependent on the specific industry and the desired re-use applications for this water, various regulatory considerations apply. The pertinent sections of the Pas-teurized Milk Ordinance (PMO) that regulate water recovery from milk and milk products in the United States are noted below as these apply to the cited case study.

• Appendix D. Standards for Water Sources
  • V. Water Reclaimed from Milk and Milk Products
• Appendix G. Chemical and Bacteriological Tests
  • Private Water Supplies and Recirculated Water - Bacteriological

According to these regulations, the three (3) general categories for reclaimed water use are:

The PMO provides very specific instruction on the criteria for use of recovered water. Highlights revolve around microbiological quality and organic load. Allow-ance for chemical treatment is provided to ensure microbiological as well as or-ganoleptic quality of the recovered water, although heat treatment is often em-ployed as a treatment method that avoids chemical additives.

Conclusions

RO Polishing of evaporator condensate and/or Reverse Osmosis permeate af-fords the opportunity of producing a category type 1 water for re-use, the most cost beneficial of the three.

As water resources, particularly those sources of high quality, become increas-ingly scarce and expensive, food processors that generate water as a conse-quence of their production processes will be well served to consider a recovery, treatment and re-use scheme that suits their on-going business strategy. Com-pounding, and quite probably over-shadowing, the water availability issue is the issue of effluent management and abatement. Both the costs of and the con-straints on effluent discharge from a processing facility will continue to challenge the on-going operations and threaten growth opportunities.

An integrated collection, treatment and re-use program employing Reverse Os-mosis polishing technology can provide a solution to those that are, by coinci-dence and not design, in the water production business.