Wastewater Treatment Secondary Treatment

After primary treatment, further treatment for the removal of colloidal and soluble organic matter present in wastewater. Biological processes are employed here to remove the remaining colloidal and soluble organic content as shown in figure 1. The treatment system consists of ASP activated sludge process (an aeration basin with return sludge facility as shown in figure 1 –a), or trickling filter a basin with fixed-filter media and secondary settling tank (SST) also known as the secondary clarrifier in conventional treatment plant as shown in figure 1b.

Theory of Organic Matter Removal Removal of colloidal suspensions is achieved by the principle of physiochemical absorption and by enmeshment of suspended solids or particulate matter on the biological floc. Reduction of soluble organic solids (BOD or COD) is achieved by microbial biosorption and their further degradation stabilization by microbes. The microbes convert these solids into simpler products like water (H2O) and carbon dioxide (CO2) and synthesize their own new cells. These cells are known as biomass or biological floc, hiaving specific gravity greater than that of water, it settles easily by gravity, therefore the new cells produced are removed in the secondary clarifier and the settled floc is known as biological

sludge or secondary sludge . the efficient performance of a biological treatment unit depends on developing a suitable mixed culture of micro-organisms in the treatment unit (bioreactor), maintaining appropriate environmental conditions for the system and removing the excess sludge produced. If excess sludge which itself is organic matter is not removed from wastewater, it will be measured as increased in BOD or COD in the final effluent and ultimately when disposed into a stream will deplete the dissolved oxygen of the receiving stream.

Activated Sludge Process (ASP) Figure 2 shows the process flow diagram for a typical ASP. The essential features of the process are: an aeration stage, solid-liquid separation following aeration and a sludge rcycle system. Wastewater after primary treatment enters an aeration tank where the organic matter is brought into intimate contact with the sludge from secondary clarifier. This sludge is heavily laden with micro-organisms which are in active state of growth. Air is introduced into the tank, either in the form of bubbles through the diffusers or by surface aerators. The micro-organisms utilize the oxygen in the air and convert the organic matter into stabilized, low energy compounds such as NO3,

SO4, CO2 and synthesize new bacterial cells. The effluent from aeration tank contained flocculent microbial mass, known as the sludge is separated in settling tank. In the settling tank the separated sludge exists without contact with the organic matter and becomes activated. A portion of AS is recycled to the aeration tank as a seed, the rest is wasted. If all AS is recycled then the bacterial mass would keep increasing to the stage where the system gets clogged with solids. It is, therefore necessary to waste some of micro-organisms and this wasted sludge is the one which is processed and disposed.

Activated Sludge Process (ASP) Basic Theory and Design In the AS system the major design parameter is the loading or the amount of organic matter (food) added relative to the micro-organism (activated sludge) available. The ratio is known as food-to-micro-organisms ratio (F/M). Unfortunately , measurements of either F or M accurately is difficult and hence, the ratio is usually expressed as the amount of BOD utilized per unit mass of active biological solids. The combination of the liquid and micro-organisms in the aeration tank is known as “mixed liquor” and the suspended solids are called “mixed liquor suspended solids MLSS”

 

(ASP) Design of Different Units and Modifications A number of design configuration and modifications of the activated sludge process are available to treat domestic and industrial wastes, depended on the intended goal of treatment i.e. whether to remove only organic content or to remove nitrogen and /or phosphorous along with organic content. Two major types of ASP which are mostly used in the field are: Conventional ASP and Complete Mix ASP

Conventional ASP The effluent from the PST (also known as settled effluent) fed to the aeration tank is mixed with return sludge at the inlet end of the tank. The oxygen demand by the micro-organisms is more in the initial length of the tank. This demand increases with the stock load near the inlet end. So synthesized biomass is also more near the inlet end but decreases towards the outlet end. Therefore hardly approach the relatively constant equilibrium condition will occur.

The complete mix system classified as follows: Complete Mix ASP The process is designed that effluent from the PST is mixed throughout the entire tank instantaneously , because of complete mixing , the organic loading is considered uniform throughout the aeration tank and the concentration of reactor biomass is not effected by shock loadings. Therefore , oxygen demand and microbial growth are also assumed as constant throughout the reactor. The complete mix system classified as follows:

Complete Mix with Aeration Only The system consists of an aeration tank only as shown below. The untreated waste is continuously fed to the reactor and is completely mixed with the biomass in the aeration tank The concentration of both the organic content and active biomass in the reactor and effluent are the same and both are a function of influent waste concentration and aeration time. Application of such system found in the aerated lagoons.

Complete Mix with Entire Excess Sludge Wasting in Effluent The system consisting of aeration sedimentation with sludge return line is considered as one unit for process analysis. Part of settled sludge is directly returned from the bottom of clarifier and excess sludge is wasted with the effluent. Therefore, the effluent SS are not equal to MLSS. It is designed to operate in the endogenous phase of growth at low F/M ratio with long aeration time. The removal of organic content is a function of flow rate of water, rate of metabolism and concentration of influent organic matter. It is effective where low BOD in the effluent is desired. The application of such system is found in the extended aeration systems like oxidation ditch

Complete Mix with Separate Wasting of Excess Sludge This is the most common sludge system consisting of aeration and sedimentation tanks in which wasting is done separetly from sludge return line as shown below: The effluent will have a slightly less flow due to separate wasting of sludge and will contain SS having minimum concentration. All normally employed conventional and complete mix AS systems are examples of this systems.

Design Consideration The design of activated sludge process requires the consideration of following significant aspects: The quantity or characteristics of raw wastewater to be treated. The desired quality or characteristics of effluent or treated wastewater. The type of reactor that will be used Volumetric and organic loading that will be applied to the reactor. The amount of oxygen required and aeration system that will be provided to apply oxygen and support mixing.

Design Consideration The quantity sludge that will be generated and wasted for its further management .

Design Criteria for Activated Sludge Process The major criteria that are usually assumed for the design of activated sludge plants are as follows: The number of aeration tanks, N= MINIMUM 2 (for small plants) and usually = 4 or more (for large plants) Depth of wastewater in the tank= 4.5-7.5 m (for diffuse aeration), = 1.0-6.0 m (for surface aeration) Free board = 0.3-0.6 m ( for diffuse aeration) = 1.0-1.5 m (for surface or mechanical aeration) For rectangular aeration tank, L:B=5:1 (for each channel for large plants) , B:D=3:1TO4:1 (depends on the aeration system).

Design Criteria for Activated Sludge Process Air requirement: 20-55 m3 of air/kg of BOD removed for diffuse aeration when F/M>=0.3 70-115 m3 of air/kg of BOD removed when F/M<=0.3 - Power required for complete mixing :10-14 kW/1000 m3 of tank volume for surface aeration system.

Problem 1 An activated sludge plant is to be designed for 10 MLD domestic wastewater flow to operate at 10 days MCRT and 6 hours of HRT. Assuming BOD5 AT 20OC as 175 mg/L in influent to the aeration tank, sludge wasting flow equal to 70 m3 /d and returned sludge concentration equal to 8000 mg/L, determine the concentration of MLVSS to be maintained in the aeration tank to achieve effluent BOD5 of 30 mg/L. also determine the recirculation ratio at which plant should be operated. Assume the kinetic coefficient kd = 0.06 /d and y=0.6.

Problem 2 Compute the mean cell residence time and recirculation ratio and volume of a reactor from following data to design a conventional ASP. Compare the recirculation ratio if the effluent biomass concentration of 15mg/L is considered in the mass balance analysis.

Problem 3 Assuming mean cell residence time of 10 days , compute the returned sludge concentration for the following given data used to design a conventional ASP for the treatment of domestic wastewater

Problem 4 Assuming the food to micro-organisms ratio equal to 0.25 and hydraulic residence time of 6 hours, compute the value MLVSS to be maintained in the reactor of a conventional activated sludge plant designed to treat 5 MLD settled wastewater with 20 mg/L of BOD5