Activated Sludge & Lagoons | Environmental Engineering - Civil Engineering (CE) PDF Download

ACTIVATED SLUDGE PROCESS

Activated sludge process is used during secondary treatment of wastewater. Activated sludge is a mixture of bacteria, fungi, protozoa and rotifers maintained in suspension by aeration and mixing . In this process, a biomass of aerobic organisms is grown in large aerated basins. These organisms breakdown the waste and use it as their food to grow themselves.  Activated sludge processes return settled sludge to the aeration basins in order to maintain the right amount of organisms to handle the incoming "food".  Activated sludge processes have removal efficiencies in the range (95-98%) than trickling filters (80-85%).  

WORKING OF ACTIVATED SLUDGE SYSTEM  

•  A primary settler (or primary clarifier) may be introduced to remove part of the suspended solids present in the influent and this reduces the organic load to the activated sludge system. 
• The biological reactor or aeration tank is filled with a mixture of activated sludge and influent, known as “mixed liquor”. It is necessary to maintain certain mixed liquor suspended solid (MLSS) in the aerated tank maintain good removal efficiency.   
• The aeration equipment transfers the oxygen necessary for the oxidation of organic material into the reactor, while simultaneously introducing enough turbulence to keep the sludge flocs in suspension. 
• The continuous introduction of new influent results in a continuous discharge of mixed liquor to the secondary settler where separation of solids and liquid takes place.
• The liquid leaves the system as treated effluent, whereas some part of the sludge is recirculated to the aeration tank called as ‘return sludge’ and rest of sludge is taken for anaerobic digestion.  

DESIGNING OF ACTIVATED SLUDGE SYSTEM 

Suppose, Q is the flow rate of influent (m³/d), QW is the flow rate of waste sludge (m³/d), Q_r is the flow rate of return activated sludge (m³/d), V is the volume of aeration tank (m³), S₀ is the influent soluble substrate concentration (BOD g/m³), S is the effluent soluble substrate concentration (BOD g/m3), X_o is the concentration of biomass in influent (g VSS/m³), X_R is the concentration of biomass in return line from clarifier (g VSS/m³), X_r is the concentration of  

Problem 4.3.1: An activated-sludge system is to be used for secondary treatment of 15,000 m³/d of municipal wastewater. After primary clarification, the BOD is 170 mg/L, and it is desired to have not more than 25 mg/L of soluble BOD in the effluent. A completely mixed reactor is to be used, and pilot-plant analysis has established the following values: hydraulic detention time (θ_C)=10 d yield coefficient (Y)=0.5 kg/kg, kd=0.05 d^-1. Assuming an MLSS concentration of 4500 mg/L and an underflow concentration of 12,000 mg/L from the secondary clarifier, determine
 (1) the volume of the reactor,
 (2) the mass and volume of solids that must be wasted each day, and
 (3) the recycle ratio. 

Solution: Given that Q=10,000 m³/d, θ_C =10 d  

Using  

 If the concentration of solids in the underflow is 12,000 mg/L 

For X_e =0

PONDS AND LAGOONS 

Other than activated sludge processes, ponds and lagoons are most common suspendedculture biological systems used for the treatment of wastewater.  A wastewater pond, alternatively known as a stabilization pond, oxidation pond, and sewage lagoon, consists of a large, shallow earthen basin in which wastewater is retained long enough for natural purification processes. Classification of lagoons is based on degree of mechanical mixing provided. 

Aerobic lagoon: The reactor is called an aerobic lagoon, when sufficient energy is supplied to keep the entire contents, including the sewage solids, mixed and aerated. To meet suspendedsolids effluent standards, solids are removed from the effluent coming from an aerobic lagoon. 

Facultative lagoon: In facultative lagoon, only enough energy is supplied to mix the liquid portion of the lagoon, solids settle to the bottom in areas of low velocity gradients and proceed to degrade anaerobically and this process is different from facultative pond only in the method by which oxygen is supplied. Facultative lagoons are assumed to be completely mixed reactors without biomass recycle . 

Aerobic lagoons with solid recycle: The aerobic lagoon with solids recycle is same as extended aeration activated-sludge process, but an earthen (typically lined) basin is used in place of a reinforced-concrete reactor basin. It is necessary that the aeration requirement for an aerobic lagoon with recycle must be higher than the values for an aerobic flow-through lagoon to maintain the solids in suspension.

DESIGN OF LAGOONS 

Process design considerations for flow-through lagoons 

• BOD removal 
• Effluent characteristics 
• Temperature effect 
• Oxygen requirement 
• Energy requirement for mixing
• Solids separation 

70% being soluble. The reaction coefficient k is 0.23 d^-1 at 20°C, and the value of temperature coefficient is 1.06. Prepare a preliminary design for a facultative pond treatment system for the community to remove 90% of the soluble BOD.  

a) Find volume of facultative lagoon to remove 90% of the soluble of BOD.
 b) Find the dimensions of three square lagoons in series with depth 1.5 m. 

Solution: (a) Estimation of rate constants at given temperature

(b) Estimation of volume of lagoon 

Summer: 

Winter: 

(c) Estimation of dimensions of three square lagoons in series