GETM

Nutrient Crisis: Wastewater Treatment Technologies and Limitations

Author discusse briefly the nutrient crisis and a few possible wastewater treatment technologies. Review by Sudhakar Viswanathan

Nitrogen and phosphorus are the nutrients of interest in water discharging into sensitive water bodies such as the Chesapeake Bay. It is estimated that nitrogen in excess of 400 million pounds is introduced each year into the Chesapeake Bay alone, mainly from agricultural runoff and wastewater plants from 7 northeastern states. Major contributors of nutrients to receiving water bodies are non-point sources; agriculture alone accounts for over 60 percent of the bays pollutants. Poor watershed management, complex farm economics and lack of models to identify individual non-point contributors have passed the burden of clean up to known point sources such as wastewater treatment plants.

Millions of dollars are invested each year to upgrade wastewater plants. Assessing treatment feasibility and economics of mitigating the nutrient crisis warrants an evaluation of available technologies and their limitations.

Some wastewater treatment plants (WWTP) discharging into the sensitive receiving bodies are required to reduce nitrogen pollutants loads in excess of 60 percent. Only a handful of technologies claim to guarantee the required discharge limits, which in some cases are as low as 3 mg/L total nitrogen. A brief summary of available technologies is presented below, while the list of available treatment options are not limited to the list below, selections were made based on popular acceptance of such technologies.

*AS: Activated Sludge (AS) process is commonly used by most WWTPs for biological treatment. This process usually involves large aeration basins (anoxic, but well mixed in case of denitrification), with highly active biomass in suspension, consuming organics and nutrients indigenous to wastewater. Although technologies like integrate fixed film/activated sludge (IFAS) and moving bed biological reactor (MBBR) are gaining popularity, we will limit this summary to the more simpler AS process. Simpler AS plants only help reduce biochemical oxygen demand (BOD), but can be adopted to reduce ammonia and nitrates. This process is limited by the rate of operation, the plants are usually large in size and require numerous stages of treatment to get to the 3 mg/L limit if applied.

*BAF: Biological Aerated (or Anoxic) Filter (BAF) combines filtration with biological carbon reduction, nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occurs in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either in upflow or downflow configuration depending on design specified by manufacturer. Popular among the BAFs are downflow sand filters with attached biomass systems treating nitrate rich wastewater. This process is limited by the rate of operation, large footprint and requires a process called bumping to release trapped gases in the media. Alternatively, BAFs designed in the upflow manner eliminate these limitations and are increasingly accepted.

*MBR: Membrane Biological Reactors (MBR) are of growing interest and are fast becoming the treatment option for small wastewater plants. MBRs includes a semi-permeable membrane barrier system either submerged or in conjunction with an activated sludge process. This technology guarantees removal of all suspended and some dissolved pollutants. The limitation of MBR systems is directly proportional to nutrient reduction efficiency of the activated sludge process. The cost of building and operating a MBR is usually higher than activated sludge or a BAF process.

While there is no ‘one technology-fits-all’ option, the general rule for selecting the best technology for a given plant depends on three key aspects- space, capital and personnel skill.

*For plants with limited land and limited capital: BAF, cost is the deciding factor, personnel training to familiarize with new treatment may be required

*For plants with ample land for expansion and limited capital: AS, cost is the deciding factor

*For plants with limited land and ample capital: MBR (or BAF)

*For plants with ample land and ample capital: All of the above, other considerations may apply

Finally, sustainability and ecological impacts of these processes should be considered prior to selection. For example, simpler technologies like AS have relatively low impact as compared to the other options. Another consideration should be the amount of synthesis the components of these processes undergo prior to use, for example choosing a BAF with sand or clay over polystyrene, or membrane technologies that have limited lifetime and may include materials that from non-renewable sources like petroleum. Also, a thorough feasibility study should be performed prior to selecting the best treatment option for a given plant. This might include, but is not limited to, an onsite pilot study to evaluate technology feasibility, a lifecycle cost analysis and impact assessment for each alternative.

Posted by Moderator on January 9th, 2006 filed in review 1 Comment »