Contaminated wastewater is one of the most problematic and potentially expensive issues for industry today. Hauling wastewater can be expensive and may expose the company to substantial liability. On-site processing can involve extensive fees and time-consuming paperwork. Fortunately, there is a solution. Evaporation is a practical, environmentally sound, and economical long-term solution to reducing wastewater and its associated risks.
For almost 100 years, natural gas pipelines have been synonymous with growth, innovation, high standards, and environmental consciousness. In the mid 90’s, one of the largest volume gas transmission companies in the United States chose to begin addressing a problem of long standing.
A common problem for many natural gas compressor stations is accumulation of standing wastewater. At one facility (in Tennessee), where average annual rainfall was approximately 50 inches, storm water run-off, and ground water seepage caused standing water to accumulate in the basement of the compressor station. Adding this to other extraneous water (e.g., condensate from air compressors), amassed unwanted water at an average rate of 2,000 gallons per week. The water, contaminated with oil and chemicals from equipment, cannot be pumped out and discharged onto the ground because of the negative environmental implications.
For many locations disposing of dirty water such as this has always been a burdensome and expensive process.
The pipeline’s Solution #1: Carbon filtering, followed by discharging water back into the environment, had been the industries’ purification method of choice because of its low cost. However, filtering left the company still owning both the water and its contaminants; thus requiring a secondary process for disposal of the water and the carbon filters. And, as the pipeline found, chemicals like glycol cannot be filtered out of the water. The pipeline found that although carbon filtering is an inexpensive water purification method, the discharged water produced from this method did require a permit from the National Pollutant Discharge Elimination System (NPDES). Even with this, filtering/discharging was a satisfactory solution until the turbine compressor cooling water systems were converted to ethylene glycol antifreeze. Since glycol cannot be removed in the filtration process, any amount of leakage contaminates the entire discharge of water.
Once this conversion had taken place, the compressor station was forced to turn to Solution #2: Hauling to an off-site disposal center. Commercial wastewater haulers remove water and haul it away for processing, typically at a publicly owned treatment works (POTW). Because gas transmission compressor stations are often in remote locations, the water may have to travel several hundred miles before disposal is possible, further increasing disposal cost. Additionally, trucks carrying contaminated water across public highways are at risk of accidents. Originators of wastewater can be liable for clean-up costs because of existing cradle-to-grave legislation. Using a commercial wastewater handler to haul the water for off-site disposal for several years proved efficient, but very expensive. The cost for hauling wastewater eventually reached a high of 80 cents per gallon.
At a typical annual volume of 103,000 gallons, the annual cost for wastewater disposal amounted to $82,400. And, as already stated, water disposal through discharge into the environment mandates an NPDES permit. Applying for one is an involved process that includes testing and permitting fees, and up to several months to complete. Once complete, the permit requires perpetual monitoring to assure that the discharged water meets strict specifications. And, there is a “Catch-22” in this process: analysis of discharged wastewater is performed at the time of discharge, which means the damage has already been done and contaminants have already been released into the environment. Wastewater discharges must then cease until the problem is resolved. Additionally, the station risks fines (from as little as $500 to as much as $25,000 per day) until the problem is corrected. What’s worse, the discharged wastewater once again can become seepage!
When wastewater is not to be discharged, a different method of disposal must be found. The next most expedient way, considering the costs and concerns associated with hauling, is Solution #3: Evaporation. This was evaluated and considered the optimum technology for disposal. It offered numerous advantages. Foremost was that of achieving superior separation of clean water from its contaminants (typically, oils and glycols). Evaporation technology permanently disposes of the now-clean water as it concentrates contaminants for secondary processing. In this way, evaporation substantially reduces the amount of wastewater that must be hauled.
In 1995, one of the pipeline’s stations was selected to be the first to purchase and install a “Samsco Water Evaporator”. The unit was installed at a total cost of $56,700. This included $32,000 for the evaporating unit itself, and $24,700 for a 12 ft. by 16 ft. metal building on a concrete slab to house the evaporator, a 1-1/2 horsepower sump pump, and valves and piping to channel the water into the unit.
Compared to the station’s prior annual cost of $82,400 for hauling wastewater, this one-time expenditure was recovered in cost savings over a brief seven months. In the years following installation, other than once replacing a corroded blower housing and vent stack there have been no additional costs associated with the unit’s performance. Today there are a number of evaporator installations throughout the nation’s gas transmission network, including many more in this one pipeline’s territory.
Wastewater at the selected station is pumped from the basement via sump pumps and deposited into one of three above ground holding tanks at the plant. These tanks range in volume from 4,500 to 10,700 gallons. From these tanks, the water is pumped into the 350-gallon tank in the “Samsco” unit and is evaporated at up to 33 gallons per hour. In this way, the evaporator is able to process up to 300,000 gallons annually when running at full capacity.
The evaporator turns the water into vapor, leaving behind contaminants in the form of slurry. Typically, fifty to seventy gallons of slurry is removed from the evaporator every six to eight weeks. This is a concentration rate of 200x (resulting in less than ten gallons of slurry for every 2,000 gallons of wastewater).
The concentrated residue is combined with other contaminated material from the site and is hauled away for incineration at minimal cost. Over a recent five-year period, the average annual cost to dispose of this residue has been as low as $2,000 to $2,500. Additionally, the station uses a small amount of defoaming chemistry occasionally needed to allow evaporation without foam-caused high-level interruptions.
The cost to operate the “Samsco” system is almost entirely that of the energy used by the evaporator. Since the unit does not require an operator and needs only occasional maintenance attention, labor costs are essentially nonexistent. The remainder of the time, automated controls assure disposal of water at a remarkably low cost.
The Operations Technician, who oversees operation of the evaporator at the referenced site, describes the equipment as virtually maintenance free. “Maintaining this particular evaporator takes only about two hours per week of my time. Cleaning it takes about three hours once every six to eight weeks”, he reported.
As an industry standard rate, evaporation cost is typically estimated at twelve cents ($0.12) per gallon. In the case of gas transmission companies, thanks to the nature of their business, a mere four to five cents worth of natural gas will eliminate a gallon of wastewater.
Evaporators give the stations control over wastewater dilemmas. For example, when an extraordinary amount of rain has filled up holding tanks, and more rain is predicted, the tanks must be emptied—fast. Without an evaporator, the station is at the mercy of waste haulers, who may be unavailable for many days. Wastewater might then spill onto the ground, with the resultant potential harm to the environment and fines to the company. Obviously, any scenario in which water accumulates faster than its disposal rate is a troublesome one.
For companies considering an evaporator investment, Samsco offers important guidelines. Apart from introducing the technology, Samsco’s product support group tests samples of the site’s typical wastewater to understand its contaminant nature and chemistry. The group performs a bench-scale pilot study to evaluate the expected performance and efficiency of the evaporation equipment to be placed at the specific location. The volume of water to be processed and the peak rate of its accumulation are projected. The system at the referenced station handles a modest volume of wastewater. Other single-module evaporator installations handle streams as small as a thousand gallons a year up to millions of gallons. Samsco provides a report of the study to the prospective user along with recommendations for installation and other equipment required to complete the operation (e.g., transfer pumps and holding tanks).
Samsco offers two types of evaporators: vacuum distillation units and smaller, simpler, thermal evaporators. Efficiency is key in selecting which unit is most appropriate to a particular location. In the location described here, reclaiming the water was not an objective, so a thermal evaporator that put the water to atmosphere was the appropriate choice.
Environmentalists for the pipeline have recommended the installation of Samsco evaporators at other company plants (and are in the process of installing more themselves). An observation they offer companies considering an evaporator system is to buy a unit large enough to allow adequate processing during peak rainfall periods. This will avoid their being forced to pay unnecessary hauling costs due to heavier than normal rainfalls that drop more water at one time than the machine is able to handle before their storage space is exhausted, which might mean holding tank overflow risks as well as the need for hauling the excess water.
For all practical purposes, purchase and installation of a Samsco evaporator form a one-time expense that easily can be recovered in cost savings, possibly within just a few months, as in the case of this pipeline’s experience. Unit operating expenses, including energy, maintenance, and manpower, are negligible. If contaminated rainwater is a compressor station’s continuing problem, a Samsco evaporator system offers a logical, environmentally sound, and economical answer.
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