Activated Sludge

In an activated sludge process, the important variables in quantifying the sludge produced that must be wasted daily are the amount of substrate (BOD or COD) removed, the mass of microorganisms in the system, and the non-biodegradable inert suspended solids in the influent to the system. Figure 2.4 shows a typical activated sludge system with these and other variables noted.

The variables can be assembled into two simple equations as follows:

Px = net growth of biomass expressed as volatile suspended solids

Y = gross yield coefficient, kg/kg or lb/lb

So = influent substrate (BOD or COD), kg/d or lb/d

S = effluent substrate (BOD or COd), kg/d or lb/d kd = endogenous decay coefficient, d-1

X = biomass in aeration tank (MLVSS), kg or lb

where

HRT = Hydraulic Retention Time

Be - Solids Retention Time (SRT)

(Also known as Meal Cell Residence Time on Sludge Age)

Influent Q - Flow

S„ - Substrate (BOD or COD) M„ - Total Suspended Solids (TSS) l„ - Inert (Nonvolatile) TSS

Aeration Tank

V - Volume M - Mixed Liquor Suspended Solids (MLSS) X-Biological Solids (MLVSS)

Final Clarifier

Return Activated Sludge (RAS)

Waste Activated Sludge (WAS)

Figure 2.4 Schematic of a typical activated sludge system.

TABLE 2.1 Typical Kinetic Coefficients for the Activated Sludge Process

Coefficient

Unit

Range"

Typicala

Y

g VSS/g BOD

0.4-0.8

0.6

Y

g VSS/g COD

0.3-0.6

0.4

kd

g VSS/g VSS • d

0.04-0.14

0.1

Values listed are for 20°C.

Values listed are for 20°C.

WAS = total waste activated sludge solids, kg/d or lb/d I0 = influent nonvolatile suspended solids, kg/d or lb/d Et = effluent suspended solids, kg/d or lb/d

Equation (2.3) dates back to 1951 (U.S. EPA, 1979). To use equation (2.3), the values of Y, the gross yield coefficient, and kd, the endogenous decay coefficient, need to be known. Table 2.1 shows typical values for these coefficients.

Solids Retention Time Solids retention time (SRT), also known as mean cell residence time, or sludge age, is the average time the sludge solids stay in the system, expressed in days. It is an important design and operating parameter for the activated sludge process. As the solids inventory in the clarifier and the solids lost in the effluent are small, SRT can be defined as the solids in the system divided by the mass of solids removed per day. It can be expressed as

Px biomass (VSS) removed per day, kg/d or lb/d

For wastewater with no nonbiodegradable volatile suspended solids (VSS) in the influent to the aeration tank, secondary biomass production and SRT have the following kinetic relationship:

where Yobs is the observed yield coefficient (kg biomass produced/kg substrate removed).

Equations (2.3) and (2.4) can be combined and simplified by using the observed yield (net biomass yield) coefficient and the flow rate as follows to quantify the sludge to be wasted daily:

In U.S. customary units, the equation is

WAS = Q [YobS (S, - S) +10 ] x 8.34lb/mgd • mg/L (2.8)

where

± obs

= observed yield, g biomass/g (lb biomass/lb) substrate removed

Q

= influent flow, m3/d (mgd)

So

= influent substrate concentration, g/m3 (mg/L)

S

= effluent substrate concentration, g/m3 (mg/L)

Io

= influent nonvolatile suspended solids concentration, mg/L

Observed yield coefficients are often reported in the literature. Figure 2.5 graphically illustrates the observed yield coefficients (VSS production) versus SRTs in an activated sludge system. As SRT increases, Yobs decreases due to biomass loss by endogenous respiration, and thus the biomass to be wasted from the system decreases. Therefore, the costs of sludge handling can be reduced by using a higher value of SRT in the design of the activated sludge system. However, the lower costs might be offset by higher costs for the increase in the aeration tank volume needed.

Nitrification Nitrogen in municipal wastewater occurs predominantly in the form of ammonia nitrogen and organic nitrogen. Approximately 60% of the total is in ammonia form and approximately 40% is in organic form; less than 1% will be inorganic nitrogen in the form of nitrate or nitrite. Nitrification is the biooxidation of ammonia nitrogen and organic nitrogen to nitrate by the

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PRIMARY TREATMENT @ 60% TSS REMOVAL 30% INERTS IN PRIMARY EFFLUENT TSS

PRIMARY TREATMENT @ 60% TSS REMOVAL 30% INERTS IN PRIMARY EFFLUENT TSS

0.6 0.8 1 1.5 2 3 4 56 789 10 15 20 SOLIDS RETENTION TIME, SRT, days

(a) With Primary Treatment

30 40

DOMESTIC WASTEWATER TSS/T0TAL B0D5 - 0.9 -1.1 INERT TSS-50% COD/BOD5 = 1.9 - 2.2

1 2 3 45678910 15 20 30

SOLIDS RETENTION TIME, SRT, days

(b) Without Primary Treatment

Figure 2.5 Sludge production in activated sludge system. (Reprinted with permission from WEF, 1998.)

staged activities of the autotrophic species Nitrosomas and Nitrobacter. Compared with processes designed for carbonaceous oxidation only, nitrification processes operate at long solids retention time. As a result, an activated sludge system in nitrification mode produces fewer waste solids than a conventional system produces. However, there is an additional component in a nitrification system, which is the net yield of the nitrifying biomass. This is estimated to be 0.15 kg of biomass per kilogram of total Kjeldahl nitrogen (ammonia plus organic) removed. This is only a small fraction of the total biomass produced. For example, wastewater that contains an ammonia nitrogen concentration of 20 mg/L and an organic nitrogen concentration of 10 mg/L would add only (20 + 10) x 0.15 = 4.5 mg/L of nitrifying biomass per liter (38 lb/MG) of wastewater.

WAS Concentration The volume of sludge produced is directly proportional to the dry weight of solids concentration in the waste sludge stream. There are basically two methods of wasting solids in a waste activated sludge treatment system. The most common method of wasting solids is wasting from the secondary clarifier underflow. Such waste activated sludge can vary, in practice, across a range from 0.4 to 1.5% total suspended solids. The second method is wasting solids from the mixed liquor. Although not commonly practiced in traditional activated sludge systems, it is becoming more prevalent in MBR applications. It has been argued that this method of wasting should improve control of the process. When mixed liquor is wasted in a traditional activated sludge system, waste sludge is at the same concentration of the mixed liquor suspended solids: about 0.1 to 0.4%. The disadvantage of this method is that to obtain a given weight of solids to be wasted, a large volume of mixed liquor must be wasted because of the low concentration of solids in it. This issue is avoided or reduced in MBR applications where mixed liquor suspended solids are much higher: in the 0.8 to 1.5% range.

The primary factor that affects WAS concentration is the settleability of the solids in mixed liquor fed to the clarifier. Various factors that affect the settleability of solids include the following:

• Biological characteristics of solids. These characteristics can be partially controlled by maintaining a particular solids retention time in the aeration tank. High concentrations of filamentous organisms can sometimes occur in mixed liquor, resulting in poor settleability of solids. These organisms can be reduced in number through control of oxygen, control of solids retention time, or sometimes by periodic chlorination of the return activated sludge to destroy the filamentous organisms.

• Sludge volume index (SVI). The SVI is defined as the volume of 1 g of sludge solids after 30 minutes of settling. The SVI is determined by placing a sample of mixed liquor of known solids concentration in a 1-L cylinder and measuring the settled volume after 30 minutes. Then the SVI is computed using the equation

mixed liquor suspended solids (mg/L) '

An SVI value of 100 or lower is considered a good settling sludge, which is sometimes called old sludge. SVI values above 100 represent young sludge. Mixed liquor with an SVI value above 150 settles poorly, possibly due to filamentous growth. • Sludge density index (SDI). The SDI is determined in the same way as the SVI and is computed using the equation

SDI = MLSS (mg/L per mL of sludge settled in 30 min) = 100/SVI (2.10)

An SDI value of 1.0 is ideal. • Surface overflow rate. The surface overflow rate of the clarifier is related to the zone settling velocity, which is the settling velocity of the sludge-water interface at the beginning of the sludge settleability test that is described in standard methods. The surface overflow rate is then determined using the equation

where

OR = surface overflow rate, m/d Vi = zone settling velocity, m/h 24 = conversion factor from m/h to m/d SF = safety factor, typically 1.75 to 2.5

This method of determining the surface overflow rate takes into account the effect of MLSS concentrations. The value of Vi will decrease, resulting in a higher clarifier surface area and better settling velocity.

• Solids flux. The solids flux is the solids in the mixed liquor divided by the clarifier area (kg/m2-d). High rates of solids flux require that clarifiers be operated at lower solids concentrations (the same effect as that for the lower zone settling velocity described above).

• Limits of flow distribution and sludge collection equipment. Poor velocity dissipation of the flow distributed to the clarifiers can result in solids carryover and poor settling of solids. Also, because of the pseudoplastic and viscous nature of WAS, some sludge collectors are not capable of reliable operation at higher underflow sludge concentrations.

• Higher sludge concentration with raw wastewater. If raw wastewater is fed to the activated sludge process instead of primary clarifier effluent, a higher sludge concentration usually results. Chemicals added to the mixed liquor for removal of phosphorus and suspended solids will also result in heavy suspended solids and higher sludge concentration.

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