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| An installation at a purified terephthalic acid plant in Taiwan. |
By Al Cocci, Connie Smith, and Robert Landine, Paques ADI Inc.
Industries are facing tougher effluent limits each year and often find it hard to know the best treatment method to meet their needs. A couple of years ago Paques, of The Netherlands, and ADI, of Canada, decided to form a company that would offer these industries a wider range of technologies to choose from; thus, Paques ADI Inc. was born.
Not all industries have the same wastewater characteristics, and there is no "one technology fits all" available on the market. There is, however, a range of anaerobic treatment technologies that will effectively meet the needs of industries having small flows and limited space. There are also anaerobic technologies for those industries with larger flows but still constrained by space, as well as those with large flow and ample space to install on-site pretreatment.
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| An installation at a purified terephthalic acid plant in Taiwan. |
Low-rate anaerobic
Some technologies are especially successful on particular waste streams. A prime example is low-rate anaerobic pretreatment. This technology is very successful at treating dairy and dairy-type wastewaters that are high in fat, oil, and grease (FOG).
One industry that has benefited from the patented low-rate anaerobic ADI-BVF digester technology offered by Paques ADI Inc. is a candy bar manufacturing facility in Canada. This plant has a design average flow of 1,135 m/d, average BOD of 1,600 mg/L, average TSS of 410 mg/L, and FOG of 200 mg/L. The on-site wastewater treatment plant now allows the manufacturing facility to meet effluent limits of 300 mg/L BOD and 350 mg/L TSS.
In addition to the low-rate anaerobic digester, the treatment package is comprised of an insulated equalization tank ahead of the anaerobic reactor, feed pumps and in-line grinder, a magnesium hydroxide feed system for pH-alkalinity adjustment, and a control building. The effluent discharges into a street sewer and flows a short distance to the publicly owned treatment works (POTW).
The entire system is controlled through a PLC system which can be accessed to change operating variables via the operator's PC workstation in the control building.
Another successful installation is at a dairy in Virginia, USA. This wastewater treatment plant was delivered on a design-build basis. ADI Systems Inc., Paques ADI Inc.'s design-build partner, provided complete services. The treatment facility comprises a screen, raw waste pumping station, equalization tank, calamity tank, an ADI-BVF digester, and a proprietary ADI-SBR (sequencing batch reactor).
This raw wastewater was a challenge as the incoming pH ranged from 2-12, the temperature ranged from 60-88°F, average TSS was 1,000 ppm, and peak day FOG concentrations approached 1,200 ppm. Nevertheless, a high-quality effluent is being produced with a BOD of 30 mg/L, SS of 30 mg/L, and FOG of 20 mg/L. The effluent discharges into a sewer and flows by gravity three miles to the POTW. The POTW effluent discharges into a creek that flows into the Potomac River and on to Chesapeake Bay.
These low-rate systems can be designed as in-ground basin reactors or as above-ground tanks, depending on site restrictions.
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| A reactor installed at a kraft mill in Alabama where it is treating foul condensates. |
High-rate anaerobic
A technology proven to be particularly suitable for treating chemical and chemical-type wastewaters is the high-rate anaerobic ADI-Hybrid. This technology combines features of the upflow anaerobic sludge bed and upflow anaerobic filter processes and has been successfully employed at several complex chemical plants around the world. One successful installation is at a purified terephthalic acid (PTA) plant in Taiwan.
In this case, the pretreatment system consists of two ADI-Hybrid reactors, one being 5,000 m in volume and the other, 4,000 m. They are designed to operate in series and give excellent performance in treating this complex wastewater.
Preliminary treatment includes screening, equalization, and pH adjustment for PTA solids dissolution prior to treatment in the anaerobic reactors.
The pretreatment facility is designed to handle 7,800 m/d of this difficult-to-treat wastewater at a design COD concentration of 6,500 mg/L (50,500 kg COD per day), including 8,400 kg/d of terephthalic acid solids. The hybrid reactors remove the majority of the biodegradable organics in the wastewater and convert them into biogas. The biogas is captured and utilized in process boilers at the site.
The pretreated wastewater dis-charges to an existing deep-shaft activated sludge treatment facility for final polishing. Hybrid reactors are suitable where land is limited.
Ultra-high-rate anaerobic
A very popular technology is the ultra-high-rate anaerobic BIOPAQ -IC (internal circulation) reactor. This system has become, for some, the preferred technology over the standard UASB (upflow anaerobic sludge bed).
The reactor features a two-stage separation/collection of biogas within a tall cylindrical vessel and uses the gas-lift principle to induce internal circulation of treated effluent.
This technology is particularly appealing to pulp and paper industries and breweries. One such reactor is successfully installed at a kraft mill in Alabama where it is treating foul condensates. The foul condensates originate from the pulp digesters and evaporators and contain mainly methanol, reduced sulfur compounds, and some terpenes.
In this case, the kraft mill was faced with having to remove methanol from the condensates to comply with the MACT I portion of the Cluster Rules. This 6.5 m diameter reactor (water depth of 20 m) typically removes more than 98 percent of the methanol at a hydraulic retention time of six to seven hours. The granular biomass inside the IC reactor converts methanol into methane and carbon dioxide; this biogas is separated inside the reactor and recovered. The biogas contains approximately 85 percent methane and is used in the mill's combo boiler.
The COD, which is typically between 4,500 and 5,000 mg/L, is reduced by 75 to 80 percent. The anaerobically treated condensates are discharged to the mill's aerated lagoons.
The tall cylindrical design of this reactor makes it very suitable for applications where land is at a premium.
Summary
As stated, one technology does not fit all wastewater treatment needs, and industries should investigate all their options before committing to a major capital investment. Every situation is unique, and a pilot study should be considered whenever there is a question as to the applicability of a technology or the treatability of the waste.
On-site pretreatment with an anaerobic system that can produce biogas for reuse in the plant as a means of offsetting energy requirements should be given serious consideration.
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