The Overlooked Oasis

Can economic recycling of wastewater help reduce water shortages?

• By Peter S. Cartwright, PE, George R. Alther
• Sep 01, 2004

Water shortages have become a problem in America due to an ever-increasing population and a dwindling water supply. Areas where water is being restricted are the west coast states, the southwestern states, and even states in New England.

Frequently, people are restricted in their use of lawn sprinklers in these states; and it will become worse unless some measures are taken to increase the water supply.

This article attempts to address these fundamental questions: (1) Are there sources of water that are untapped? (2) Can water be recycled by industry at a cost that does not cripple smaller users? The answer to both questions is yes.

Let's first look at the regulatory environment, particularly regulations pertaining to the discharge of oily wastewater by industry to publicly owned treatment works (POTW). The current limit for industry for the discharge of oily wastewater by the U.S. Environmental Protection Agency (EPA) is 100 milligrams per liter (mg/L). This limit is often based on the different industries that generate these wastewaters.

There are no known data on how many wastewater industry discharges contain oil and grease, but the number is substantial. Large water users are laundry processors, refineries, steel mills, car and truck washes, military bases, construction sites, and so on.

Discharge Limits
Let's suppose that EPA decides to lower discharge limits for oil and grease to 10 mg/L or even 5 mg/L. Would that cripple many industries because they would have to install expensive treatment systems? In Minnesota and Ohio, the standards are already at 10 parts per million (ppm).

Furthermore, there are industries that need a near zero level of oil and grease in order to prevent fouling of the media that remove heavy metals and hazardous organic compounds from water, such as the plating industry. These industries are surviving. What will industries do if they have to treat the water to pass such stringent oil and grease limits? They will obviously start recycling their wastewater, lowering the cost of use and discharge of fresh water.

The question is, are there technologies available to reach such standards at an economical cost? The answer is yes.

Oil/water separators have been in use for many years, but they are not sufficiently reliable to guarantee reduction to below 10 ppm consistently, so fines for violations of the discharge limits are a possibility. On the other hand, organically modified clays, which remove small amounts of oils from water, are being used, and are permitted by EPA and the Army Corps of Engineers. Thus, in its simplest form, a treatment train of an oil/water separator, a bag filter, and an adsorber filled with organoclay are all that is needed. If the oil is chemically emulsified, treatment becomes more complicated, but systems to break emulsions are well established.

The Cost of Water
Let's now look at the cost of water. In Minneapolis and St. Paul, Minn., the Metropolitan Council is responsible for the wastewater treatment plant. In addition to the "connection fees," the user fee is $134 per 100,000 gallons. This does not include charges of $88 per 100,000 gallons for the treatment of "high-strength" wastewater, for concentrations for chemical oxygen demand (COD) exceeding 500 mg/L and 250 mg/L total suspended solids (TSS). For drinking water, charges are an additional $231 per 100,000 gallons, and for sewer discharge a charge of $300 per 100,000 gallons. In other cities, such as Elgin, Ill., the same costs are $420; in Las Vegas they are $980.

Suppose you are in charge of a linen service in a hotel in Las Vegas, and you use 24 million gallons per year (gpy). Your annual cost is $235,200. Converting the amount of water used to gallons per minute (gpm), you need a system that can handle about 44 gpm, 24 hours per day, seven days a week. Due to the presence of soaps and soils, the system requires methods to break chemical emulsions. An oil/water separator that can handle such a flow costs around $10,000. An adsorber to house the organoclay, probably a unit that can handle at least 2,000 pounds or 40 metric feet (ft³)of organoclay, costs another $10,000. Ancillary equipment, which could be an ozonation unit to break down the emulsion, and an activated carbon adsorber to remove the surfactants, may be another $20,000. This adds up to a total of less than $50,000 initial capital cost. Annual costs include maintenance and replacement of the organoclay and activated carbon, which adds $20,000 per year.

The bottom line is, if the facility can recycle one half of its wastewater, it saves $116,000 per year. The one time capitalization cost is recuperated in less than one year. That means the hotel saves itself $116,000 per year does not have to worry about inspectors entering its facility (and incurring possible fines) and the city of Las Vegas has 12 million gallons of extra water available every year.

A second example might be a steel mill in St. Paul, Minn. The mill might use 2 million gallons per year, at a cost of $12,000 per year. If this water is flowing 24 hours per day, seven days a week, the flow rate is 4 gpm. This is a very small system. Assuming spikes in flow, one would propose an oil/water separator-organoclay system to handle 20 gpm. Such a system would cost about $10,000, meaning that it is paid for within one year. If the steel mill cuts its water use to 1 million gallons per year, the annual maintenance of the system will be about $7,000, the citizens have an extra million gallons available, and the steel mill adds a small amount of money to its bottom line. If these quantities of water saved per year are multiplied across the country, it is obvious that hundreds of millions of gallons could be saved.

To extend this to the use of stormwater, the following scenario is envisioned: A refinery, for example, or a parking lot, sets up a catch basin to collect stormwater, treats it, and uses it as a fresh water supply in its operation, further saving water from the municipality. In states such as Florida, sinkholes could potentially be lined and used to collect stormwater.

Combining Technologies to Move Toward a Solution
Oil/water separators are well known in the industry and have been in use for many years. But the lynch pin of the system is the organoclay, which is not as well known but has been on the market since 1985. Organoclays consist of bentonite, which is modified with a quaternary amine. Combining the two by means of ion exchange renders a product that can be used to remove smaller amounts of oil from water. It does this by the amine chains partitioning into the oil droplets, which are then fixated.

The organoclay is blended with anthracite, which has a similar bulk density, to prevent early plugging of interstitial pores. The organoclay/anthracite is capable of removing more than 50 percent of its weight in oil, seven times as much as activated carbon. That means 2,000 pounds of organoclay can remove 1,000 pounds of oil. It is a granular media, placed into the same types of filter vessels as activated carbon. No special handling is necessary. Disposal as a nonhazardous waste is usually into a dumpster or a landfill, as long as it is not considered hazardous waste due to the removal of volatile organic compounds (VOCs) such as benzene.

Based on this treatise, it is clear that:

• The water supply in the United States can be increased at no additional cost and that industry can recycle a good portion of its wastewater at no extra cost.
• The technology to do so is available and economical. If municipalities agree to extend tax breaks to ease the capitalization costs, all the better.

This article originally appeared in the 09/01/2004 issue of Environmental Protection.