How can 100% compliance with discharge standards be guaranteed when building a wastewater pretreatment system? That was our first concern when we began planning a new wastewater treatment system for the Blue Grass Tank Wash in Blue Grass, Iowa.
Blue Grass Tank Wash had a continuous-flow-through wastewater treatment system that was installed more than 15 years ago. It consisted of an initial pH adjustment and oil skimming with subsequent addition of coagulant. The wastewater stream was filtered through a rotary drum vacuum filter that was pre-coated with diatomaceous earth.
With the old system, we were never absolutely certain of producing quality effluent. If the pH probe was malfunctioning, a discharge of water with a pH outside permitted limits could occur. Also, there was no way to continuously monitor the effectiveness of the coagulant. Under conditions of inadequate coagulation, suspended solids and grease would remain.
Another major concern for our facility was a need for wastewater storage because we haul wastewater to a POTW (publicly operated treatment works) sanitary district in Davenport, Iowa. We concluded that a wastewater treatment system with significant storage capacity was a major advantage for this facility. It would allow us to use the wastewater treatmentequipment as additional storage.Records of wastewater generation at Blue Grass Tank Wash over the past few years indicated a plant that could treat approximately 15,000 gallons of water per day would be adequate for the job. Typical untreated wastewater has a pH of approximately 12 and a TSS (total suspended solids) of approximately 4,000 mg/l. Oil and grease vary from a low of approximately 11 mg/l up to 1,000 mg/l.
The existing wastewater treatment system included twin 21,000-gallon wastewater storage vessels. We decided that these vessels should be retained and used for incoming and outgoing wastewater storage.
All pretreated wastewater is hauled to a municipal wastewater treatment facility, which charges by the gallon for disposal. Any system that we designed must meet anticipated future industry categorical standards that are likely to be promulgated by the Environmental Protection Agency. Therefore, pH, FOG (fats, oils, and greases), and various metals may have to meet more rigid or constricted standards. We reasoned that pH should be between 6 and 9, FOG should be less than 100 mg/l, and various metals such as zinc and chrome should be kept below 3 mg/l. We wanted a system that could be easily adapted.
One of the major obstacles to installing a wastewater treatment system was the small area of floor space available at the Blue Grass facility. The wastewater treatment equipment needed to fit into a space of less than 430 square feet. Fortunately, the wash rack has a ceiling height of over 18 feet that allowed for a tall structure.
We had seen many types of treatment technologies applied to tank wash wastewater. These included ozonation, ultraviolet catalyzed peroxide treatment, ultrafiltration, aerobic biological, anaerobic biological, air stripping, and physico-chemical (conventional) treatment technology.
We determined that conventional treatment was able to meet the discharge standards under most conditions. This system also offered the benefits of economy as well as reliability when compared with the alternatives. Other technologies could be added as a second-stage treatment at a later date.
A batch treatment system seemed to best meet the needs of the wash rack due to its reliability, but most of these systems take up too much space. Bruce decided that the space limitations could be addressed by installing a batch treatment system using flat-bottom tanks.
There are major advantages to flat-bottom tanks over those with conical bottoms. Cone-bottom tanks must be elevated and are generally much more expensive. They take up significantly more space for the same volume flat-bottom design.
Nevertheless, cone-bottom tanks have generally been considered necessary for batch treatment systems. Many wash rack managers believe in cone-bottom wastewater treatment tanks so that sludge can be removed.
Bruce found that the problem with flat-bottom tanks can be finessed. The key is to implement careful operating procedures and achieve a clear supernate that is decanted. Wash rack managers must realize that incomplete removal of sludge from wastewater treatment tanks will not incapacitate the system.
A schematic of the final design can be seen in Figure 1. Water flows into a 21,000-gallon storage tank where free oil is allowed to surface. Water then descends by gravity into one of two 8,500-gallon batch treatment tanks. Chemicals such as coagulant, acid, lime, and polymer are added directly to these treatment tanks.
After treatment, clear water is decanted and the remaining sludge is transferred to a 5,000-gallon storage tank where additional sedimentation and decantation of clear water occur. Sludge from the bottom of this tank is transferred to a filter press for dewatering and disposal.
The 16-foot-high tanks chosen for this project are great for doing large batches but present several difficult design problems. The height of each tank gave precious little space above to install a mixer. This problem was solved with a side-entry mixer with a shaft seal. The second challenge with the tall tanks is the fact that the operator cannot easily look over the top to observe the wastewater during treatment. This was solved with a six-inch-diameter glass window at eye level on each treatment tank. The windows were fitted with internal wipers to keep the glass clean.
A third problem with tall tanks was pH probe installation. The probes must not be exposed to air when the treatment tanks are emptied. However, removing them from the tank after each treatment was deemed too troublesome. The problem was solved by devising a pH probe piping loop fitted with a pump. When pH monitoring is desired, the pump is simply turned on to move water from the tank over the pH probe. These design features allow the operator to monitor the tank from ground level and greatly simplify his job.
There was a concern that treating an 8,000-gallon batch would be long and tedious for the operators. We addressed this issue by designing each treatment tank with a powerful mixer and installing pumps that could add large quantities of treatment chemical in a short time.
We installed a filter press for dewatering. Our intentions were to avoid the complication of using a filter precoat procedure or a pre-conditioning of the sludge before pressing. For this reason, we were prepared to tolerate slow pressing cycles.
To speed press cycling as much as possible, we provided sludge prethickening in the holding tank. We also decided to use dolomitic lime for pH adjustment to improve the consistency of the sludge.
Because the operators were familiar with the way the wastewater was treated with the old system, they had no trouble adapting to the new system. Initially, there were some concerns about the unautomated nature of the system. For instance, they are required to add each chemical by actuating a button on the control panel.
These concerns were alleviated within a couple of days. Total time for addition of chemicals to the 8,000-gallon batch is less than 20 minutes.
With the new system, the operator is alerted by audible alarm when a treatment tank is full and ready to be treated. The powerful agitator means pH responds quickly to additions of acid or lime. Flocculation by the polymer is easily noted through the tank window.
A two-hour settling time is allowed before decanting. Sludge level is determined by taking a series of samples from a port located down the side of each tank. Decanted water is quickly removed with the aid of a powerful centrifugal pump.
The remaining sludge is transferred to the sludge holding tank in less than 15 minutes. As a result, a total cycle time for an 8,000-gallon tank is less than five hours. It is generally not necessary to treat more than two batches a day. Total operator involvement is approximately two hours. All the operators agree that the new system is easier to operate than the old one and that it has proven more reliable.
Chemical costs compare favorably with the old system. Blue Grass Tank Wash was spending approximately two cents per gallon on diatomaceous earth alone. Total chemical cost with the old system was more than four cents per gallon.
The operating cost of the new system is much lower. Cost of chemicals for treatment is now approximately 0.5 cents per gallon of water treated. Blue Grass Tank Wash is hauling sludge in a 20-cubic-yard roll-off box that is filled about every six weeks, compared with every three weeks with the previous system.
Presently, the new system could be described as semi-automated. There are continuous level monitors in each tank that respond automatically to an overfill condition or other critical situations. However, all chemicals and transfers are initiated manually.
Initially, we felt that we would eventually fully automate the new batch treatment system to reduce labor costs. Specifically, we felt that pH adjustment and other chemical addition would be automated. However, after several months of operation we have decided that further automation might be a mistake.
The operators enjoy monitoring the system, and we think their involvement makes for a more reliable operation. We also believe that the operator time involved is not as costly as it might seem because the operator can leave during any point without ill effects on the treatment process. This allows him to work in other parts of the plant and return to wastewater treatment at a more convenient time.
Steve Berger is president of Gless Bros Inc, Blue Grass, Iowa, which owns and operates Blue Grass Tank Wash. Bruce Bishkin is owner of Autocatalytic Sales, an engineering sales and consulting firm in Elmhurst, Illinois.