Researchers at Purdue University have...
On a planet that has over 75 percent of its surface area covered with the precious resource of water, only 3 percent of the earth’s water is considered potable – and this small fraction is continuing to dwindle. Some geographic areas are worse than others. For example, in China, where over 25 percent of the world’s population will soon reside, there is less than 10 percent of the earth’s supply of potable water. The problems associated with potable water supplies have global implications. In the past, wars have been fought for less precious commodities. Yet, even though our potable water supply is extremely valuable, the vast majority is not being used for drinking, bathing, or cooking. Instead, industry and irrigation place an extremely high demand on our dwindling water resources.
The solution may be as close as the nearest wastewater discharge pipe. The demands of industry and irrigation can be easily satisfied with reclaimed water and a principle source of this water is the effluent from local sewage treatment facilities. New technologies can be cost-effectively utilized to convert this waste stream into a reusable product. A drought-proof supply of high-quality water would be created that can be utilized for a multitude of purposes. One of the key requirements of such treatment facilities is the reduction of nutrients, such as nitrogen, from the effluent water. A cost-effective solution for nitrogen reduction would allow reclaimed water to be made available in practically every city.
The Chino Basin Municipal Water District recently constructed a water reclamation facility. This plant has a capacity of 7 MGD (26,495 M3/D), with a future expansion to 28 MGD (105,980 M3/D). Effluent from this facility is required to meet the stringent requirements established by the State of California for reclaimed water. These high-quality standards included a maximum yearly average Total Inorganic Nitrogen (TIN) level of 10 mg/l.
This facility utilizes the oxidation ditch process and the raw sewage is evenly split into three parallel bio-treatment trains. Each train consists of a BULLSEYE™ nutrient removal tank, an oxidation ditch, a vertical turbine aerator, an intra-channel clarifier, and the NITROX™ denitrification system. Effluent from the BOAT® intra-channel clarifiers flows through dual-media, tertiary filters. Sludge is continually removed from the oxidation ditch using an automatic wasting system which also maintains a constant sludge age. The utilization of both the BULLSEYE nutrient removal process and the NITROX denitrification system was extremely unique, since both systems provide alternative methods for the reduction of nitrogen in the effluent.
The BULLSEYE process utilizes a triple-ring design and each zone has a different hydraulic detention time. The center zone is sized to act as a biological "selector." Each circular zone is mixed by a single, sub-surface propeller and the geometric shape eliminates the potential for dead zones and the settlement of solids. The flow between each zone must pass through a specially designed stainless steel transfer vault. These vaults force the flow from the top of one zone to the bottom of the next ring, thereby eliminating the potential of short-circuiting, which can be found in most single-tank designs. The Chino facility was the first application of this nutrient removal process in the world.
The NITROX system utilizes the entire oxidation ditch as an alternating reactor. The aerator is turned off and mixers are engaged to provide mixing. The ditch is returned to an aerated state when the system detects that the nitrates have been depleted via an ORP signal. The ORP control system provides a more accurate and reliable control mechanism as compared to the utilization of timers or dissolved oxygen levels. The NITROX system was originally developed as part of a US Department of Energy research program. One of the original intents in the development of this system was to be able to retro-fit the technology into existing treatment facilities. This would allow currently operating plants to be able to reduce the electrical costs, as well as lowering the nitrogen in the effluent. The Chino facility was the first new installation of this technology in the world.
Not only was this facility the first in the world to be designed to utilize either the BULLSEYE nutrient removal process or the NITROX denitrification system, it was also the first treatment plant to combine these patented technologies. In order to certify the effectiveness of these new system under full-load conditions, an extensive testing program was conducted by the project management firm of North American Treatment Systems. The results of this test can be found in Table 1. Since there was not adequate flow to test the full capacity of 7 MGD (26,495 M3/D), a single oxidation ditch train, with a design capacity of 2.33 MGD (8,832 M3/D) was tested. During the test period, the average influent BOD and TSS concentrations were over 75% higher than the monthly design parameters. The TKN for the test period averaged nearly 28% higher than design. Despite these conditions, the plant produced a high-quality effluent, even under adverse loading.
A second testing program was conducted which challenged the process with higher flow volumes, while maintaining influent concentrations (BOD = 254mg/l; TSS = 280mg/l) that were much closer to design specifications. This second test determined that the 7 MGD (26,495 M3/D) facility could actually be operated at a higher capacity. The results of this testing program proved that the plant could handle influent flows up to 8.1 MGD (30,659 M3/D) of domestic waste and still produce an effluent quality that meets the stringent standards for reclaimed water. A summary of the monthly effluent quality from this facility since the completion of the testing can be found in Table 2.
The Chino Facility utilizes a very effective combination of treatment processes which can generate a high quality effluent, even under adverse conditions. Nutrient removal is effectively and efficiently accomplished thereby allowing for the reuse of the effluent. Due to the resiliency and flexibility of the process, only minimal operator attention is required (maximum of 5 people working 8 hour/5 day shifts).
This combination of new technologies was also bid against a conventional activated sludge system. The bidding process proved that this unique process was far less expensive to construct. In addition, this new system required less land than a conventional system of the same capacity. This facility serves as a good example of how new technologies can be utilized to protect and conserve our available potable water supplies.