Wastewater treatment (WWT) uses many different processes to achieve the ultimate goal of discharging wastewater that meets all applicable regulations. In the Food and Beverage industry the composition of wastewater streams can be very complex, and highly variable, making the treatment of wastewater quite challenging, especially in the Secondary WWT phase. The demands made on the food and beverage industry are numerous and range from health and financial issues to environmental concerns.
Two additional factors are adding to this challenge. First, environmental discharge regulations continue to tighten, making it difficult for some facilities to consistently operate without NPDES violations. Second, climate change, drought conditions and the trend towards water conservation are forcing some WWT facilities to recycle a portion of their water for in-plant use. The end-use of this recycled water may dictate an even higher quality standard than what is required for discharge.
Wastewater treatment can be divided into five processes: pre-treatment, primary, secondary, and tertiary treatment then finally disinfection.
Secondary WWT is arguably the most important of the processes used in the treatment of waste in the Food & Beverage industry, due to the high and varying levels of soluble and suspended organic matter in the wastewater. Unfortunately, it is also arguably the most complex of the WWT processes, and can therefore create many challenges from an operations perspective.
The Activated Sludge Process
The first major development in Secondary WWT was the introduction of the activated sludge process in England in 1913. The activated sludge process combines sewage, a concentrated mass of microbes, and high levels of dissolved oxygen to promote the consumption of organic content.
The activated sludge process is still very widely used, and has evolved into many different variations, depending upon specific waste treatment requirements.
The activated sludge process can be quite daunting to operate properly. Loss of control by WWT plant operators, can result in loss of the activated sludge, decimation of the microorganism population, and ultimately in non-compliance.
It takes experience and expertise to operate an activated sludge facility in the event of upset conditions. Another issue with most traditional activated sludge processes is the large footprint and the associated high initial capital costs.
As a result of these issues with the activated sludge process, newer technologies have been developed over the past few years. The Membrane Bioreactor (MBR) and the Sequencing Batch Reactor (SBR) processes are two such technologies.
The use of MBR and SBR has become widespread in the Food and Beverage industry, due to the typical wastewater composition, a general tightening of discharge regulations, and water shortages. MBR and SBR treated wastewater is much better suited for reuse or recycle than activated sludge treated effluent.
The MBR process combines activated sludge treatment with a membrane for liquid-solid separation. While MBR can achieve almost complete separation of suspended solids and dramatic reduction in contaminants, it is prone to membrane fouling. Fortunately, the recent advent of PTFE membranes and improved system designs have minimized the need for membrane maintenance to prevent fouling.
A SBR process typically consists of at least two identically equipped reactors with a common inlet, valved to direct flow to one reactor or the other. As the name implies, the reactors work as batch operations and by nature are easier to operate than continuous flow systems since every batch can be treated and controlled separately. In addition, SBR systems often have a smaller footprint and capital cost and are easier to operate than other types of systems. On the downside, SBR may be limited to smaller wastewater flows.