ECO SAF SUBMERGED BIOREACTOR TECHNOLOGY
The principles of the Eco-SAF
Submerged aerated filters are a compact solution for biological wastewater treatment. They operate to the same basic biological principles as the activated sludge process. The bacteria required for the process are generally present within the wastewater itself although seeding is usually required when a plant is first installed. After this initial seeding, the bacterial population will usually maintain itself given that the correct conditions within the system are maintained (bioaugmentation may be required to introduce specific bacterial populations to deal with certain pollutants). Effluent is treated as the bacteria break it down to utilise the nutrients within it to grow. In essence, the challenges in running a successful SAF system are based on maintaining optimal conditions for the bacterial populations to multiply. The other key factor is the retention time through the SAF. The longer the retention time, the more treatment the bacteria are able to undertake. The Eco-SAF aims for a minimum of six hours retention time during dry weather flow.
The biological zone
In the Eco-SAF, the effluent enters at a high level at one end of the steel tank housing the biological reactor. It flows vertically down the tank wall and enters the biological cell at its base. The base is sloping in a 'V' shape to allow any solids to settle. This allows for very simple de-sludging, via valves at the base of each biological cell. Above this are the aeration disc fine bubble diffusers. The air and the effluent flow upwards into the plastic media driven by the lifting effect of the rising bubbles. The oxygen transfer rate is extremely efficient due to the fine bubbles and the shearing effect of the bubbles flowing through the media. The media used has a surface area:volume ratio of 150m2/m3 for general treatment and 240m2/m3 for enhanced nitrification. The effluent will rise through the media and weir over at a lower level than that at which it entered, into biological cell two. The effluent as before enters the cell from the base. This configuration allows the flow of effluent to be uniform throughout the system. A typical system has four biological cells some or all of which may be converted to nitrifying cells by using the denser media and heating the effluent to increase the nitrification rate. The final effluent then weirs over at the end of the tank at a lower level that that at which it entered.
The EcoSAF, in the above configuration, acts as a Plug-Flow System. This means that the effluent and oxygen mixture flow through the tank in a uniform manner. At Pollution Control (UK), we also operate a Mixed System where we use random media instead of rigid media. This means that the effluent/oxygen mixture tumbles through the tank in a random manner. We are able to adapt our system configuration to suite the specific needs of our clients.
The bacteria within the EcoSAF grow at a very rapid rate due to the optimal conditions which are maintained in the system. The system loses 'old' bacteria in the sludge, and continuously replaces them with 'new' bacterial growth. This recycling happens via a process called sloughing. The bacteria naturally adhere to the surface of the media due to the action of polysaccharides produced by the bacteria. A bacterial film then builds up as more bacteria adhere onto it. The growth of this film is supported by nutrients and oxygen dissolving through it. As the film gets thicker, less oxygen and nutrients will get through to the bacteria on the inside, which are adhering to the media. There will be a point where nothing reaches this bottom layer and the bacteria die. As a result, the whole film loses its adherence to the media and falls off and settles with the sludge. The gap left will then be colonised by a new bacterial film. This process enables for the constant renewal of the bacterial population and keeps the activity of the bacterial population at a maximum rate.
System configuration
The EcoSAF can either be installed on its own to improve an existing works or it can be part of a full system manufactured by Pollution Control (UK). This system would include a primary clarifier/balance tank, which will allow for about 30% of the solids to settle and allow for the regulation of the flow into the EcoSAF tank. This is crucial for optimal performance. The system would also include a final clarifier where the final solids are allowed to settle before the treated water flows out.
The power use for the EcoSAF plants ranges from 2.8kWh for 100pe plants to 5.5kWh for 750pe plants.
Typical site layout of an EcoSAF system.
