Comprehensive Analysis of Water Treatment Processes: Unveiling Advantages and Disadvantages
Community wastewater treatment processes are developed based on traditional urban wastewater treatment processes. Conventional urban wastewater treatment processes mainly include: SBR wastewater treatment process, CASS wastewater treatment process, A/O method, aerated biofilter, MBR method, biological contact oxidation method, etc.
1. SBR wastewater treatment process
SBR is short for Sequencing Batch Reactor, a type of activated sludge wastewater treatment technology that operates in an intermittent aeration mode, also known as the Sequencing Batch Reactor activated sludge method.
Different from traditional wastewater treatment processes, SBR technology uses time-segmented operation instead of space-segmented operation, non-steady-state biochemical reaction instead of steady-state biochemical reaction, and ideal sedimentation instead of traditional dynamic sedimentation. Its main characteristics are orderly and intermittent operation. The core of SBR technology is the SBR reactor, which integrates the functions of equalization, primary sedimentation, biodegradation, and secondary sedimentation in one tank, without a sludge return system.
Advantages of SBR process
(1) The ideal plug flow process increases the driving force of the biochemical reaction, improves efficiency, and stabilizes the operation effect.
(2) Resistant to shock loads. The retained treated water in the tank has a dilution and buffering effect on the wastewater, effectively resisting the impact of water volume and organic pollutants.
(3) The existence of DO and BOD5 concentration gradients in the reactor effectively controls the expansion of activated sludge. (4) It has a good denitrification and phosphorus removal effect.
(5) Simple process flow and low cost. The main equipment is only one batch reactor, without secondary sedimentation tank and sludge return system. The equalization tank and primary sedimentation tank can also be omitted, with compact layout and small land area.
Disadvantages of SBR process
(1) High requirements for automated control.
(2) Short drainage time (during intermittent drainage), and the drainage requires that the settled sludge layer is not stirred, so special drainage equipment (decanting device) is needed, and the requirements for the decanting device are very high.
(3) Large requirements for post-treatment equipment: such as large disinfection equipment, large contact tank volume, and large drainage facilities such as drainage pipes.
2. CASS wastewater treatment process
CASS is developed based on SBR. A biological selector is added to the inlet of the SBR tank, realizing continuous inflow (continuous inflow during sedimentation and drainage periods) and intermittent drainage. The main purpose of setting up a biological selector is to select flocculating bacteria in the system, and its volume accounts for about 10% of the entire tank. The process of the biological selector follows the substrate accumulation-regeneration theory of activated sludge, allowing the activated sludge to undergo a high-load adsorption stage (substrate accumulation) in the selector, followed by a lower-load substrate degradation stage in the main reaction zone to complete the entire substrate degradation process and sludge regeneration.
CASS has the following advantages:
(1) Low construction cost;
(3) High organic matter removal rate, good effluent quality. After filtration and disinfection, it can be reused as reclaimed water;
(4) Simple management, reliable operation, not easy to cause sludge expansion; low sludge yield, stable properties.
CASS has the following disadvantages:
(1) Winter or low temperature will affect the operation;
The A/O process consists of an anoxic tank and an aerobic tank in series, which removes organic matter and achieves good denitrification effect. A/O is also called pre-denitrification. The most significant process characteristic is that the denitrification tank is set before the carbon removal process. First, the wastewater is introduced into the anoxic tank, and the denitrifying bacteria in the returned sludge use the organic matter in the raw wastewater as a carbon source to reduce a large amount of nitrate nitrogen in the returned mixed liquid into nitrogen gas, thus achieving the purpose of denitrification. Then, it enters the subsequent aerobic tank. A sedimentation tank is set after the O section, and part of the settled sludge is returned to the A section to ensure that the A section has enough nitrate.
The advantages of this method are high treatment efficiency, simple process, low investment, low operating cost, high degradation efficiency of pollutants in the anoxic denitrification process, and high volume load. However, due to the lack of an independent sludge return system, it cannot cultivate sludge with unique functions, and the degradation rate of refractory substances is low; if the denitrification efficiency needs to be improved, the internal circulation ratio must be increased, which increases the operating cost. In addition, the internal circulation liquid comes from the aeration tank and contains a certain amount of DO, making it difficult to maintain the ideal anoxic state in the A section, affecting the denitrification effect, and the denitrification rate is difficult to reach 90%.
Aerated biofilter is an advanced wastewater biological treatment process, which integrates the advantages of activated sludge and biofilm methods, and has both biochemical reaction and physical filtration functions.
Modified ceramic filter media are filled in the aerated biofilter, and a large number of bacteria grow on the surface of the filter media, forming a certain biofilm after running for a period of time. In the process of degrading organic pollutants in the aerated biofilter, due to assimilation, a large number of new bacterial bodies grow on the surface of the filter media, making the biofilm thicker. At the same time, due to the interception of some suspended solids, the head loss of the filter increases. When the head loss reaches a certain range, it should be backwashed to wash out the aged biofilm, and the backwash wastewater flows into the equalization tank for re-treatment.
In the application of buried reclaimed water treatment projects, due to the large lifting height of the aerated biofilter, the elevation arrangement is difficult to coordinate, and it is generally used as a post-treatment process, but its elevation arrangement is still a thorny issue.
5. MBR wastewater treatment process
MBR, also known as a membrane bioreactor, is a novel water treatment technology that combines a membrane separation unit with a biological treatment unit. Membranes come in a wide variety of types. Classified by separation mechanism, there are reaction membranes, ion exchange membranes, and osmotic membranes; classified by membrane properties, there are natural membranes (biological membranes) and synthetic membranes (organic and inorganic membranes); and classified by membrane structure, there are flat plate, tubular, spiral, and hollow fiber types, etc.
Advantages of MBR technology
(1) Due to the efficient separation effect of the membrane, the separation effect is far better than that of traditional sedimentation tanks, and the effluent water quality is stable.
(2) This process has low excess sludge production, reducing sludge treatment costs.
(3) Small footprint, not subject to site restrictions
(4) Convenient operation and management, easy to achieve automatic control
Disadvantages of MBR technology
(1) High membrane cost, high infrastructure investment for membrane-bioreactors;
(2) Membrane fouling is prone to occur, which brings inconvenience to operation and management;
(3) MBR technology has high energy consumption.
6. Biological contact oxidation method
Biological contact oxidation is a biofilm treatment technology improved and evolved from the biological filter pool through contact aeration. It has the basic characteristics of biofilm methods, which can use the microbial population attached to the surface of the filler to adsorb and oxidize pollutants in the water to achieve the purpose of removing pollutants, and it is also different from other biofilm methods:
(1) The filler in the reactor is completely immersed in the wastewater to provide a habitat for microbial growth, so it is also called a submerged filter bed reactor;
(2) The oxygen supply method and intensity are different. Mechanical equipment is used to aerate the wastewater, which is different from the biological filter pool relying on natural ventilation for oxygen supply. The mass transfer rate of oxygen is high, improving the biological degradation efficiency.
The advantages of this process are:
(1) Large specific surface area. Because biological contact oxidation uses filler as a carrier, and the specific surface area of the filler is larger than that of general biofilm methods, a stable high-density ecological system can be formed, the membrane hanging period is relatively shortened, and under the condition of treating the same amount of water, the hydraulic retention time is short, the required equipment volume is small, and the site area is small.
(2) Biological contact oxidation has the characteristics of high sludge concentration and long sludge age. It has a strong decomposition ability for some difficult-to-degrade organic matter, strong system shock load resistance, and high efficiency. Relevant reports show that under general conditions, the volume load of biological contact oxidation can reach 3～10kgBOD5m-3d-1, which is 3-5 times that of ordinary activated sludge methods, and the COD removal rate is 2-3 times that of traditional biological methods.
(3) Compared with the ordinary activated sludge method, because the sludge production of biological contact oxidation is less, sludge expansion generally does not occur during operation, and it is not necessary to frequently adjust the return sludge amount and DO value.
(4) Simple equipment, easy operation, convenient maintenance, low operating cost, and low comprehensive energy consumption
Biological contact oxidation process, characteristics of biological contact oxidation
The biological contact oxidation process uses fixed biological fillers as microbial carriers, and the carriers with microbial growth are submerged in water, and the aeration system supplies oxygen to the microorganisms in the reactor. Because the microorganisms in the biological contact oxidation method are fixedly grown on the biological filler, it overcomes the shortcomings of easy loss of suspended activated sludge and can maintain a high biomass in the reactor.
Introduction to biological contact oxidation process
The main function of the screen is to intercept large debris in the wastewater to prevent damage to the pumps or process pipelines of subsequent treatment units.
In order to ensure the continuity and stability of the operation of the treatment structure, an equalization tank should be designed to adjust the water quality and quantity of wastewater to ensure the treatment effect.
3. Hydrolysis acidification tank
The hydrolysis acidification tank in this treatment process can degrade solid substances into soluble substances and large molecules into small molecules to reduce the load on subsequent treatment.
4. Contact oxidation section
The contact oxidation tank is an immersed aeration biological filter pool, with fillers in the pool. The biofilm attached to the fillers adsorbs and oxidizes and decomposes the organic matter in the wastewater. The oxygen required by the microorganisms is aerated by a blower. The contact oxidation tank has the following characteristics: large specific surface area of the filler, good oxygenation conditions in the pool, the unit volume biomass in the contact oxidation pool is higher than that of the activated sludge method aeration tank and biological filter pool, so it can achieve a higher volume load; because a considerable part of the microorganisms are fixedly grown on the surface of the filler, the operation and management are convenient; because of the large amount of fixed growth in the pool, the water flow is completely mixed, so it has a strong adaptability to sudden changes in water quality and quantity; because of the high sludge concentration, when the organic load is high, its F/M remains at a certain level, so the sludge yield can be equivalent to or lower than that of the activated sludge method.
The vertical flow sedimentation tank uses gravity separation to separate the solid and liquid phases of the wastewater. The water enters the pool from the lower outlet of the central pipe, and due to the obstruction of the baffle plate, it flows to the surroundings and is distributed on the entire horizontal section, slowly flowing upward. When the sedimentation speed exceeds the upward flow velocity of the water, the particles settle down to the sludge hopper, and the clarified water overflows from the weirs around the pool.
Characteristics of biological contact oxidation process
(1) Biological contact oxidation has strong resistance to shock load and water quality changes, and stable operation.
(2) Biological contact oxidation has high volume load, small footprint, and low construction cost.
(3) The biological contact oxidation method produces less sludge, requires no sludge return, and is simple to operate and manage.
(4) The biological contact oxidation method sometimes sheds some fine biofilms, resulting in poor sedimentation performance and slightly higher suspended solids concentration in the effluent, generally around 30 mg/L.
Applicable Scope of Biological Contact Oxidation
The biological contact oxidation method is suitable for wastewater treatment projects in small and medium-sized hospitals with a bed capacity of less than 500. It is especially suitable for situations with limited land area, small water volume, large water quality fluctuations, low pollutant concentration, difficulty in cultivating activated sludge, and convenient management.
