Secondary treatment basically is a biological treatment process to remove all the suspended solids that does not settle in the primary tank which involves microorganisms. It removes the dissolved organics which is used by the bacteria as food and energy for growth in the oil and gas industry. There are two categories of biological treatment which is the suspended growth process and attached growth process.
Suspended Growth Process
Suspended growth process is the reproduction of microorganisms from wastewater suspension. The organic constituents in the suspension is used as the food for the microorganism growth and forms active biomass. Activated sludge process is usually used in the oil and gas industry as it more effective compared to others. It is a sewage treatment where activated sludge and sewage mixture agitated. It contains the suspended colloidal, inorganic matters and dissolved organics which converts the organic materials to energy and carbon source for the microbes growth and oxidized products.
Activated Sludge Treatment
Figure 3.4.5 shows the process of activated sludge.
The bacteria is brought to the organic materials after the wastewater enters the aeration tank. Meanwhile, the air is flowed into the system continuosly in order to maintain the solid contain in the suspension. The mixture of sludge and wastewater is called mixed liquor which is in the tank whereas the mixed liquor suspended solid (MLSS) is the biomass contained. The mixed liquor volatile suspended solids (MLVSS) in the wastewater treatment is usually in the range of 80% and 10% inerts (Thompson, 1998). The outlet of the aeration tank is sent to the gravity clarifier. In the clarifier, the organics are converted to biomass and separated. Then, the concentrated sludge is recylced and the remaining sludge is disharged. The fungi also can reduce the chemical oxygen demand (COD) of the water ( Yu, Han, & He, 2017). There is also activated sludge treatment with the powdered activated carbon treatment where the carbon absorption and biological oxidation will occur at the same time which improves the removal of the contaminants from the wastewater.
Membrane Bioreactor Technology (MBR)
This system combines the membrane process and the suspended growth bioreactor. Hence, we do not need the secondary clarifiation in the system. According to Thompson, the micro filtration membranes located in a steel membrane tank which has the low pressure vacuum that pulls the water through the membrane and pump the filtered water to next process by retaining the solids. The formaldehyde, phenol and total petroleum hydrocarbon (TPH) were removed up to 96, 79 and 94% respectively ( Kulkarni, 2016 ). The MBR operates at higher MLSS. However, MBR is not recomended for this because it is very costly. Hence, the activated sludge is recomended but it needs tertiary filtration.
Figure 3.4.6 shows the membrane bioreactor teknology system.
In this aerated lagoons system, air is injected for biological treatment. There are two types of aerated lagoon which is aerobic lagoons and anaerobic lagoons. For aerobic lagoons, the settling happens at the pond which separated by the baffles where the oxygen dissolved is the same in the basins. The sludge that are settled is cleared constantly. The oxygen is maintained in the above layer of the liquid and others remains in the anaerobic lagoons. The suspended solids goes to the downstream part of lagoon and go through the anaerobic decomposition (Thompson, 1998). Aerated lagoons are not used widely in the refinery industries because of the quality is not good enough compared to the effluent quality of the activated sludge system.
Attached Growth Process
In this particular process, the bacteria is attached to an inert packing material which is rocks and plastic materials. They converts to biomass and carbon dioxide which keeps growing until it reaches one thickness.
The wastewater is flowed continuosly on the packing bed which is the rock and take away the clean water to different compartments. In the trickling filter, there would be a bacteria layer which biodegrades the organics that will clear the liquid flowing through. The final clarifier will remove the microbial growth.
Figure 3.4.7 shows the tricling filter system.
Petroleum refinery wastewater also contains high level of nitrogen. Hence, nitrification process is needed to remove all the nitrogen contains in the wastewater. There are two ways to do nitrification which is by nitrifying the bacteria or by doing nitrification and denitrifiation together. According to Kulkarni, the higher MLSS value and mixing rate of 300rpm is more effective in removing the nitrogen. If the level of nitrogen cannot be controlled in the ammonia removal water stripper we would probably need a secondary treatment of nitrification combined with denitrification. Methanol is added into the tank whereas for the other process the treated wastewater is recycled back to the aeration tank to ensure the reduction of nitrate occurs in the effluent (Thompson, 1998). As observed by Kulkarni, a high amount of ammonia oxidizing Archaea is more effective in the nitrification process. There will a tertiary treatment process if there is in need of other materials to be removed.
Figure 3.4.8 shows the nitrification process system.