8.3 Activated Sludge Process Control & Operations

Principles of Aerobic Biological Secondary Treatment

Secondary wastewater treatment relies on suspended-growth aerobic microorganisms—primarily heterotrophic bacteria—to remove soluble and colloidal organic matter that escapes primary sedimentation. In the activated sludge aeration basin, microorganisms consume organic carbon measured as 5-day Biochemical Oxygen Demand ($\text{BOD}_5$) as an energy and carbon source, converting it into carbon dioxide ($\text{CO}_2$), water ($\text{H}_2\text{O}$), and new bacterial cell mass (mixed liquor suspended solids, or MLSS).

The simplified aerobic oxidation reaction is expressed as:

$$\text{Organic Matter } (\text{C}_6\text{H}_{12}\text{O}_6) + 6\text{O}_2 \xrightarrow{\text{Heterotrophic Bacteria}} 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{New Biomass}$$

To ensure complete oxidation and prevent anaerobic septic conditions, aeration blowers or mechanical surface aerators must maintain a minimum Dissolved Oxygen (DO) concentration of $2.0 \text{ mg/L}$ throughout the aeration basin. According to South Carolina DES Regulation 61-67, conventional secondary treatment facilities must consistently achieve a 30-day average effluent concentration of $\text{BOD}_5 \le 30 \text{ mg/L}$ and $\text{TSS} \le 30 \text{ mg/L}$, with a minimum overall removal efficiency of $85\%$. The 7-day average limit for both parameters is $45 \text{ mg/L}$.

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Key Process Control Parameters & Calculations

Effective operation of an activated sludge plant requires continuous tracking of four fundamental process control parameters: Food-to-Microorganism (F/M) ratio, Mean Cell Residence Time (MCRT), Sludge Volume Index (SVI), and Return/Waste Activated Sludge rates.

1. Food-to-Microorganism (F/M) Ratio

The F/M ratio measures the daily mass of incoming organic food ($\text{BOD}_5$) applied per unit mass of active biological microorganisms (Mixed Liquor Volatile Suspended Solids, or MLVSS) in the aeration tank:

$$F/M = \frac{\text{lbs } \text{BOD}_5 \text{ added per day}}{\text{lbs MLVSS in aeration basin}} = \frac{Q \text{ (MGD)} \times \text{Influent BOD}_5 \text{ (mg/L)} \times 8.34}{V \text{ (MG)} \times \text{MLVSS } (\text{mg/L}) \times 8.34}$$

2. Mean Cell Residence Time (MCRT) / Solids Retention Time (SRT)

MCRT (also called sludge age or SRT) represents the average number of days microorganisms remain inside the treatment system before being intentionally wasted or lost in the effluent:

$$\text{MCRT (days)} = \frac{\text{lbs MLSS in aeration basin (+ clarifier blanket)}}{\text{lbs WAS SS lost/day} + \text{lbs Effluent SS lost/day}}$$ $$\text{MCRT} = \frac{(V_{\text{aeration}} \times \text{MLSS} \times 8.34) + (V_{\text{clarifier}} \times \text{Clarifier MLSS} \times 8.34)}{(Q_{\text{WAS}} \times \text{WAS}_{\text{SS}} \times 8.34) + (Q_{\text{eff}} \times \text{Eff}_{\text{SS}} \times 8.34)}$$

Operating MCRT targets vary by operational objective:

• Conventional BOD Removal (Non-Nitrifying): $3 - 5 \text{ days}$
• Nitrifying Systems ($20^\circ\text{C}$): $8 - 15 \text{ days}$
• Extended Aeration / Oxidation Ditch: $15 - 30 \text{ days}$

3. Sludge Volume Index (SVI)

SVI measures the settling and compaction characteristics of mixed liquor solids in a 1,000 mL graduated cylinder after 30 minutes of quiescent settling ($\text{SV}_{30}$):

$$\text{SVI (mL/g)} = \frac{\text{SV}_{30} \text{ (mL/L)} \times 1,000}{\text{MLSS } (\text{mg/L})}$$

• SVI $< 80 \text{ mL/g}$: Dense, rapid-settling, over-oxidized floc. Settles fast but sheds pinpoint floc, leaving a cloudy supernatant that raises effluent turbidity and TSS.
• SVI $80 - 120 \text{ mL/g}$: The target band. Well-structured floc that settles and compacts properly, with a clear supernatant.
• SVI $120 - 150 \text{ mL/g}$: Marginal. Still operable, but treat it as an early filament warning and check DO, F/M, and nutrients before it climbs.
• SVI $> 150 \text{ mL/g}$: Severe filamentous bulking. Slow-settling sludge blanket with high risk of solids loss over secondary clarifier weirs.

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Mass Balance & Pumping Rate Control (RAS & WAS)

Return Activated Sludge (RAS) Control

RAS recycles settled active microorganisms from the bottom of the secondary clarifier back to the aeration basin entrance. The required RAS pumping rate can be estimated from settleability testing or solids mass balance:

$$\text{RAS Rate } (\% \text{ of } Q) = \frac{\text{SV}_{30}}{1,000 - \text{SV}_{30}} \times 100$$ $$Q_{\text{RAS}} = Q \times \frac{\text{MLSS}}{\text{RAS}_{\text{SS}} - \text{MLSS}}$$

Conventional RAS pumping rates typically range from $25\%$ to $75\%$ of plant influent flow $Q$, while nitrifying or extended aeration plants may require $75\%$ to $150\%$.

Waste Activated Sludge (WAS) Control

WAS removes excess biological solids generated daily to maintain target MCRT and F/M ratio:

$$\text{WAS Mass (lbs/day)} = \frac{\text{Total lbs MLSS in System}}{\text{Target MCRT (days)}} - \text{lbs Effluent TSS/day}$$ $$Q_{\text{WAS}} \text{ (MGD)} = \frac{\text{WAS Mass (lbs/day)}}{\text{WAS}_{\text{SS}} \text{ (mg/L)} \times 8.34}$$

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Worked Process Control Calculation Example

Problem Statement: An operator manages an activated sludge plant with an aeration tank volume $V = 1.2 \text{ MG}$, influent flow rate $Q = 3.0 \text{ MGD}$, and influent $\text{BOD}_5 = 200 \text{ mg/L}$. Testing reveals MLSS $= 2,500 \text{ mg/L}$, MLVSS $= 2,000 \text{ mg/L}$ ($80\%$ volatile fraction), 30-minute settleability $\text{SV}_{30} = 300 \text{ mL/L}$, WAS solids concentration $\text{WAS}_{\text{SS}} = 7,500 \text{ mg/L}$, and effluent $\text{TSS} = 10 \text{ mg/L}$.

Step 1: Calculate the F/M ratio.

$$\text{Influent BOD}_5 \text{ Load} = 3.0 \text{ MGD} \times 200 \text{ mg/L} \times 8.34 = 5,004 \text{ lbs BOD}_5/\text{day}$$ $$\text{Aeration Basin MLVSS Mass} = 1.2 \text{ MG} \times 2,000 \text{ mg/L} \times 8.34 = 20,016 \text{ lbs MLVSS}$$ $$F/M = \frac{5,004 \text{ lbs BOD}_5}{20,016 \text{ lbs MLVSS}} = 0.25 \text{ day}^{-1} \quad (\text{Optimal conventional range})$$

Step 2: Calculate the SVI.

$$\text{SVI} = \frac{300 \text{ mL/L} \times 1,000}{2,500 \text{ mg/L}} = 120 \text{ mL/g} \quad (\text{top of the 80–120 target band})$$

Step 3: Calculate the daily WAS pumping rate required for a target MCRT of 10 days.

$$\text{Total Aeration MLSS Mass} = 1.2 \text{ MG} \times 2,500 \text{ mg/L} \times 8.34 = 25,020 \text{ lbs MLSS}$$ $$\text{Total Mass Wasted + Lost Daily} = \frac{25,020 \text{ lbs}}{10 \text{ days}} = 2,502 \text{ lbs SS/day}$$ $$\text{Effluent TSS Mass Lost} = 3.0 \text{ MGD} \times 10 \text{ mg/L} \times 8.34 = 250.2 \text{ lbs TSS/day}$$ $$\text{Required WAS Mass} = 2,502 - 250.2 = 2,251.8 \text{ lbs WAS SS/day}$$ $$Q_{\text{WAS}} = \frac{2,251.8 \text{ lbs/day}}{7,500 \text{ mg/L} \times 8.34} = 0.0360 \text{ MGD } (36,000 \text{ gpd})$$

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Operational Troubleshooting & Biological Pathology