Wastewaters generated by municipalities and industries must be adequately treated prior to discharge to ensure a healthy natural environment. In conventional biological treatment of wastewaters with low concentrations of biodegradable contaminants, air is pumped into the system to enable microbes to carry out the task of breaking down the wastes. However, when the waste materials are too concentrated, the cost of delivering air to the system increases significantly, and this method therefore becomes much less cost-effective.
The Wastewater Biotreatment Group (WBG) at the Department of Civil Engineering, NUS, has been working on an alternative treatment system - Anaerobic Sequencing Batch Reactor (anSBR) for treating highly-concentrated wastewaters. In contrast to the conventional aerobic biotreatment technology, anaerobic treatment (as the name suggests) does not need oxygen (as in air). In addition, the process generates less biomass and at the same time produces methane gas, which can be recovered for energy. This method of wastewater treatment is therefore more cost-effective for highly-concentrated wastewaters. The challenge, however, is to find a cost-effective method of building such a system and an easy way of operating it.
The WBG has extensive experience in designing and operating the aerobic SBR. Members of the group have developed numerous full-scale SBR plants for application across the Asian region and are also probably among the few worldwide who have developed full-scale anSBRs (18 units) for treating high strength wastewater since the late 1980s. In an effort to better understand the anSBR process and to push anSBR technology to a higher level of application, the WBG has teamed up with a R&D Group from the Department of Civil Engineering, University of Toronto, to undertake R&D on the anSBR. The researchers at the University of Toronto have considerable experience in modelling treatment processes. In this connection, the research group at the University of Toronto has developed a computer simulation package for the anSBR, which can be used to assess the effects of wastewater composition and operational conditions on the performance of an anSBR system.
With the aid of the computer simulation model, one can analyse different functional conditions quickly and economically. The information generated from the simulation model can assist in proposing the most cost-effective design - giving the anSBR process a competitive edge in real world applications. The computer model allows one to obtain useful information within minutes that would otherwise need months in the laboratory. It enables one to predict the best way to design and operate an anSBR. The results obtained can then be verified experimentally. Such a computer-aided approach would best be used as a screening tool that would enable one to focus the experimental effort within the predicted region and thus help to save considerable time and money to establish the desired design and operational protocols.
The research team from the National University of Singapore has meanwhile conducted extensive research on lab-scale anSBRs. It has been noted that:
1. The anSBR is potentially an effective process for treating chemical industrial wastewater containing inhibitory organic compounds. Chemical oxygen demand (COD) removal efficiency will could be above 90% even at F/M ratios higher than 0.6 gCOD/gVSS.d with an influent COD concentration of 10,000 mg/l.
2. The system is able to tolerate shock loading at F/M ratios higher than 1.0 gCOD/gVSS.d and continued to achieve a satisfactory effluent quality.
3. The polymer enhancement technique has a significant effect on biomass granulation as evident from the earlier appearance and larger size of granules (Figure 1).
4. The time-oriented nature of the anSBR provides the flexibility of adjusting operating conditions such as Hydraulic Retention Time (HRT) or Organic Loading Rate (OLR) by simply re-setting the feed or decant volume or re-setting the sequence time. This flexibility is not possible in a continuous flow system.
The team has developed the key design, operational and control parameters, and created a granular sludge medium which can work effectively even at high chemical waste concentrations. Being able to control granulation allows one to enhance the anSBR’s effectiveness. With such an operational strategy, the system start-up can be completed within a shorter time and without the need for preformed granular sludge. At this juncture, the WBG is at an advanced phase of setting up a pilot-scale anSBR in-situ for the treatment of high strength chemical wastewater.
The work by both teams have complemented each other well, producing several cost-competitive engineering developments that would help speed up greater commercial application of the anSBR. This project is funded by the National Science and Technology Board (for the NUS study) and the Canadian Government (for the University of Toronto study). In addition, the studies are supported by Biotreat International (Singapore) and CH2M Gore & Storrie (Canada).
Assoc Prof WJ Ng, Tel: 874 2172/2102, Fax: 779 1635, Email: email@example.com