The Scranton Army Ammunition Plant (SCAAP) in Scranton, PA, is one of the few industrial facilities capable of forging large caliber projectiles used by the military. To keep the hot (2300°F) freshly forged projectiles from sticking to the forge, a mineral oil based lubricant that has graphite suspended in it is used to lubricate the forge. The spent forging oil along with cooling water collects in trenches under the forges and is sent to an oil water separator (OWS), and the recovered sludge is landfilled. However, the OWS functioned poorly and the concentration of oil in the discharge water often exceeded the permitted limit. During the course of the project, SCAAP installed a skimmer that captures much of the oil, which is recycled. However, even after skimming, the concentration of oil in the water exceeds the discharge limit permitted by the Scranton Sewer Authority.
In addition to Scranton, treatment plants, wash racks, fuel depots, industrial operations, and maintenance facilities at U.S. Department of Defense (DoD) activities annually generate millions of gallons of wastewater contaminated with thousands of tons of oily sludge. Collecting and disposing non-recyclable oily sludge is increasingly costly and time consuming. In the Navy, the yearly operation and maintenance (O&M) costs associated with OWSs and bilge oily wastewater treatment system (BOWTS) units are estimated to be $24 million, and the Army estimates that the cost for disposing of oily sludge generated at wash racks alone is $150,000 per base. In the civilian sector, the U.S. Environmental Protection Agency (USEPA) estimates that oily sludge disposal costs $2 billion per year. As an alternative to the current practice (landfill disposal), which is increasingly costly and restricted, on-site bioremediation offers attractive cost savings and eliminates long-term liability associated with landfill disposal.
Since oily waste is composed of refined petroleum hydrocarbons, most of which are biodegradable, on-site treatment of oily waste is technically feasible and has been confirmed in lab and pilot-scale tests. Most importantly, bacteria capable of degrading oily waste are already present in the waste; thus one of the primary requirements for successful treatment is to create conditions that optimize the growth and activity of the indigenous hydrocarbon-degrading bacteria. The most direct approach, which was used at SCAAP, is simply a well-mixed tank or sequencing batch reactor (SBR) into which oily waste (the primary food source) is fed. To ensure that the water-insoluble oil is easily accessible to the bacteria, it is mechanically emulsified and the reactor is supplemented with inorganic (nitrogen, phosphorous) and organic (vitamins and amino acids) nutrients which make it easier for the bacteria to grow. The addition of the organic nutrients also supports a more metabolically diverse population of hydrocarbon-degrading bacteria. To further promote growth, a near neutral pH is maintained and an aeration system provides oxygen and helps keep the SBR mixed.
Ideally, oily waste should be burned or re-refined; however, the physical chemical characteristics of this material are not compatible with currently available reuse technologies. Thus, DoD and the civilian sector are faced with recurrent and escalating costs for land-filling oily waste, which is in addition to the cost of removing it from the waste stream. Furthermore, DoD remains liable for the material once it is landfilled. Since on-site biological treatment does not require separation prior to treatment, these costs (which can be considerable) are reduced if not eliminated and once the waste is degraded, it is no longer a liability. Compared to the recurrent cost of land filling, biological treatment is cost effective and the payback period can be as short as 1 year.
The objective of the project was to demonstrate and validate an innovative application of bioreactors for the on-site treatment of oily sludge generated at DoD activities. More specifically, it was shown that:
- Reactor was easily assembled on site using commercial components.
- Operation was optimized to treat the oily sludge.
- Design, cost, and performance data were developed.
The two primary quantitative performance objectives, which were both met, were: (1) the design and operation of the reactor would permit the oily waste to be degraded within design time to or below the discharge limits; and (2) the use of the reactor would reduce costs and the payback compared to the current practice of approximately 3 years.
The oily sludge biodetoxification system consists of a bioreactor tank, receiving or holding tank, pH controller, aeration and mixing system, ultra-filtration unit, and volatile organic compound (VOC) filters. Air and nutrients (fertilizer) are fed into the bioreactor tank where organisms already present in the sludge degrade the oily sludge, leaving only biomass, carbon dioxide, and clean water for recycling or discharge.
The initial performance as indicated by the concentration of oil in the treated water met the discharge requirements. However, unexpected problems associated with the physical properties of the oily sludge and the scale of the system required that the system be modified, and SCAAP also reduced the volume of wastewater but not the volume of oil. The treatment system was modified to accommodate these problems, and subsequent testing demonstrated that the concentration of residual oil in the wastewater was reduced to the permitted discharge limits and a simple carbon canister (rather than the originally proposed biofilter) was sufficient to remove VOCs in the SBR exhaust air.
During the course of the project, three implementation issues arose: (1) SCAAP reduced the volume of cooling water which increased the concentration of oil beyond the design treatment capacity. This was addressed by installing a skimmer to recover the oil, which is purchased by a recycler. (2) Oil pooled on the surface of the reactor, which limited bacterial accessibility and created impossibly long treatment times. (3) Pooled oil congealed on the surface and sunk to the bottom of the reactor where it accumulated. The last two problems were solved by installing a weir at the surface of the SBR that collected the pooled oil before it could congeal. This oil and water were recirculated through a centrifugal pump, which kept the oil mechanically emulsified and readily available to the bacteria. Concurrently, it was recognized that the aeration system was not adequate and the new air headers were fabricated and installed. These modifications enhanced mixing, which improved degradation and reduced the potential for the oil to congeal and accumulate on the bottom of the SBR. Shortly after the project was completed, SCAAP substituted a water-based lubricant for the previously used mineral oil lubricant that has proven to be biodegradable, and they replaced the tube filter with a membrane filter that has enabled them to use all of the treated wastewater for plant cooling.