FALL/WINTER 1997

MSS Teams Up With Krofta

Once again, MSS has aligned itself with another industry leader. Krofta® Engineering Corporation, based out of Lenox, Massachusetts, is the world’s leader in Dissolved Air Floatation (DAF) technologies for municipal and industrial water and wastewater treatment. This is why we opted to use a Krofta unit in our first Secondairy® system.

As you may remember from previous newsletters, Secondairy® is a process that recovers solids from a plant’s wastewater stream. It was developed by MSS researchers and received patent approval earlier this year.

Although other companies manufacture DAF units, MSS selected to use Krofta’s unit for several reasons. Worldwide, Krofta has more than 2000 DAF units in operation involving industrial and municipal wastewater treatment and sludge thickening, pulp and paper, food processing, laundry, chemical, and oil removal applications. Furthermore, their Superacell™ is cost effective, continuous, liquid/solid separation equipment. Because of its compact design, the unit saves both space and installation costs – it was an ideal choice for use in our Secondairy® process.

Abstract

Reverse Osmosis (RO) has gained wide acceptance in industry, replacing applications formerly reserved for evaporation. Continuous improvements in membranes, systems and controls have further opened the field of application for RO. The primary advantages of RO over evaporation are: reduction of the need for supporting boiler capacity, low building height, and lower upfront capital expenditures. New developments promise to make RO competitive even for applications requiring high concentration in the future.

Discussion

Water removal is perhaps one of the oldest industrial processes. Until three decades ago, evaporation was the only available concentration process. The advent and refinement of reverse osmosis membranes and related systems allow the industry to consider an alternative process. In some cases, a concentration process is practical with RO whereas it was not practical with an evaporator. In most cases, RO is less capital intensive than an evaporator is. In almost all cases, RO requires less energy input to achieve the same amount of work as an evaporator does.

At the present state of RO technological development, RO is usually not cost effective or practical for concentrations above 25%. Evaporation remains the most practical method of concentrating to high solids. This results in the common practice of using an RO unit to remove most of the water from a fluid – up to 25%, then "finishing" the concentration process on an evaporator.

The dairy industry had many evaporators for concentrating milk and whey. Once RO technology entered the picture, many expanding plants elected to increase capacity by adding RO units rather than replacing their evaporators with larger capacity evaporators. This improved energy efficiency while minimizing capital outlay.

The introduction of RO polishers now makes it practical to directly reuse permeate and/or condensate within the plant in place of fresh water. This advent of "closed loop" technology helps greatly to meet new stringent environmental standards.

In general, evaporation results in BOD wastewater quality slightly better than RO. Whereas RO permeate BOD quality degrades over the life of the membranes, evaporator condensate remains essentially constant over the evaporator life. Since neither RO permeate nor evaporator condensate is acceptable – as is – for direct reuse, both usually require polishing for full recovery. Therefore, the slightly better condensate quality available from an evaporator is not usually a significant advantage.

The chemical composition of condensate and permeate is somewhat different since condensate is the consequence of distillation and permeate is the result of molecular size separation. Permeate does not contain suspended matter whereas, condensate may contain suspended matter, especially if the evaporator is equipped with inefficient vapor liquid separators. Acids and volatile chemicals will end up as condensate from an evaporator and they are usually found in permeate.

Which to Use: Evaporation or Reverse Osmosis?

Each application is somewhat different. However, there are common criteria for making a rational and economic decision. The criteria generally are:

• Size, type and operating cost of existing evaporator (if applicable)
• Volume (per hour, hours per day, days per week) and starting solids of material to be concentrated
• Required concentrate solids
• Cost for alternative systems
• Steam cost
• Electrical and electricity
• Membrane replacement costs and predicted frequency of replacement
• Cleaning costs for alternative systems
• Depreciation costs for alternative systems
• Building costs for alternative systems
• Boiler capacity and costs to expand (if applicable)
• Permeate utilization situation
• Need for sanitary or non-sanitary design
• Pre-treatment requirements and costs
• Heating and cooling requirements
• Distance to customer and related freight costs at various solids levels

Based on historical dairy plant applications, some practical advice is possible:

• A small cheese plant which has no existing evaporator should look at RO if the intermediate solids (<25%TS) concentrate (retentate) can be delivered at a profit to a larger dairy with further processing capacity.
• A cheese factory (with an existing evaporator) that needs additional capacity should compare RO pre-concentration to evaporator expansion and to evaporator replacement. RO expansion probably will be the best economic alternative.
• Where applicable, pre-concentration of skim milk by RO ahead of cheese making is much more economic than pre-concentration of whey.
• An upgraded control system, proper product pre-treatment (fines and fat removal), and completely reliable C.I.P. chemicals are essential parts of an RO system.
• Evaporation is presently the only practical process for concentrating beyond 20 to 25% total solids. Small plants, which must concentrate to high solids, are usually better off to pre-concentrate to 20 or 25% TS by RO and finish the concentration on a small used evaporator. This will change with the introduction of new technology.

 

A Typical Medium-Sized Plant Comparison

In the following operation cost comparisons, we will look at three evaporator scenarios and one RO scenario to concentrate 40,000 pounds per hour of 6.2% solids whey to 22.5% solids and to 45% solids. The comparison assumptions are: Steam cost $4.50 per 1,000 lbs., electric cost 40.06 per kWh, operating hours per year 5,000, and one year membrane life.

 

		COSTS PER HOUR
					Cost/Lb. H20
	Steam	Electric	Membranes	Total	Removed
Old 3-Effect Evaporator	$42.40	$ 3.60	$0.00	$46.00	$.0015
New 5-Effect Evaporator	$20.84	$ 4.20	$0.00	$25.04	$.0009
New MVR Evaporator	$ 7.20	$15.20	$0.00	$22.94	$.0008
RO to 22.5% TS	$ 0.00	$10.80	$12.00	$22.80	$.0008
Finisher for RO to 45%	$11.25	$ 1.20	$0.00	$12.45	$.0022
RO + Old Finisher	--	--	--	$35.25	$.0010
New Finisher to 45%	$ 5.63	$ 1.80	$0.00	$ 7.43	$.0013
RO + New Finisher	--	--	--	$30.23	$.0009

*The lowest operating cost system to achieve 22.5%TS is the RO. This is also the lowest installed cost system.

*However, when considering concentration to 45%TS, the best investment is not clear. The lowest operating costs are with an old evaporator, which will cost almost the same as a new RO plus an old finisher, depending on site-specific conditions. Only a complete study of all site-specific costs will determine the best solution for concentrating to 45%TS.

Peripheral Equipment, Facilities – Other Considerations

Evaporators require taller buildings, vacuum pump systems, cooling tower and water pumping systems, large boiler capacity (or large electric motor driven compressor), and a C.I.P. system. These evaporator support systems and the tall building increase the project cost considerably. They represent a significant increase in processing system complexity and maintenance. RO systems usually are supplied complete. RO systems do not require the sophisticated support equipment or C.I.P. tanks to recover cleaning reagents. In general, it could be said that RO systems are simpler machines than evaporators are.

However, membranes can be irreversibly fouled if operated or cleaned incorrectly. A membrane system’s capacity decreases over the daily run and over the membrane’s life. More careful product pre-treatment to remove fines or fat and to adjust temperature is required to achieve successful RO operation.

Conclusion

If concentration to <20 – 25% is being considered, RO is almost certainly the right answer. However, if concentrating to solids higher than 25%, the issue requires careful study of all possible options.

Visit Us on the Web!

MSS’ website is ready for viewing and we invite you to visit us at http://www.mss-inc.net. Our site contains both general and technical information. We hope you will find the information helpful, and we encourage you to visit us periodically to see what’s new.

MSS Gets Breathing Room

In an attempt to better serve you, we have expanded our facilities. Until recently, our office space was very compact. As the number and scope of projects continued to grow, our facilities became increasingly inadequate. Furthermore, in order to house our additional personnel, conduct research, process orders, construct equipment, and maintain accurate records, we simply needed more breathing room.

With increased floor space, we now have designated areas for research and development, controls and panel construction, as well as shipping and receiving. Our research laboratory is where ideas for new applications are put to the test. Our pilot units allow us to simulate large scale operations for only a fraction of the cost, in turn, saving our customers both time and money. Similarly, our controls department is now able to work on several projects simultaneously, while performing system tests and program evaluations prior to on-site implementation. Finally, as a result of our expansion, we can more effectively process and track orders in our shipping and receiving department.

Thus far, the expansion has made working conditions much more bearable, to say the least. Thank you for giving us the opportunity to serve you. We look forward to working with you in the future, and we are ready to meet all of your needs!