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Aeration Basins & Blowers Controls

Control Strategies PIDs
Control Loop Descriptions
Aeration Basin Liquid Flow P&ID
Aeration Basin Air Flow P&ID
 
Control Variables

Proper control of the operating parameters must be maintained at all times to successfully operate an activated sludge system. To do this, a good working understanding of these parameters is essential so potential problems can be corrected prior to a process upset. The following operating parameters should be considered:
 
      · Number of units in service and hydraulic retention time (HRT)
      · Solids retention time (SRT), waste activated sludge (WAS) wasting rate, and solids production rate
      · Food-to-microorganisms (F/M) ratio (related to SRT)
      · Respiration rate (related to SRT)
      · Return activated sludge (RAS) flow rate
      · Mixed liquor recycle (MLR) flow rate
      · Anoxic/anaerobic selector volume and HRT
      · Aeration process dissolved oxygen (DO) concentration

Number of Units in Service and HRT
The number of units in service controls the HRT which is affected by influent flows, recycle flows and aeration volume. The HRT is important because it is one of the parameters controlling the mixed liquor suspended solids (MLSS) concentration in the aeration basins and the resulting solids loading to the secondary clarifiers. The operator has only limited control over influent flows (as described under Non-controllable Variables). The operator can control the RAS (i.e. recycle) flow rate within the bounds of good process control. The operator can also control aeration volume by taking units in and out of operation. Number of units in service is the primary means of HRT control.

Solids Retention Time and Sludge Wasting Rate
The solids retention time SRT is the primary parameter that should be used to control the effluent quality and amount of activated sludge solids in the aeration system. This method is recommended instead of the traditional food-to-microorganism method for the following reasons: 

      · SRT does not depend on the 5-day BOD (BOD5) test; therefore, a 5-day waiting period is not required 
        to obtain control information. 
      · The SRT method allows the aeration basin to find its own equilibrium MLSS concentration. With reasonably 
        stable influent conditions, it usually takes a time period equal to approximately 3 times the SRT for equilibrium to
        be reached. This method does not force the system to operate at an improper MLSS concentration. 
      · A higher SRT usually implies a lower F/M ratio with higher MLSS levels in the aeration basins and vice versa.

The mass of solids removed from the activated sludge system has two components: 

      · WAS - Waste Activated Sludge, sludge that is intentionally wasted from the system; 
      · ESS - Effluent Suspended Solids, the suspended solids contained in the effluent that passes over the weirs of
        the secondary clarifiers.

With properly operating clarifiers, wasting through WAS is typically significantly higher than wasting through ESS. The WAS rate is considered to be the system's solids production rate. The WAS is routed to solids storage and handling facilities for further processing.

Solids Production Rate, Y
The solids production rate may be used to estimate the apparent yield of an activated sludge system in terms of pounds of total suspended solids produced per pound of total BOD removed. Because of influent variability, this parameter can vary considerably in the short term. To have diagnostic value, it must be based on plant performance over an extended period of time (average over at least one week of stable operation). Most activated sludge systems produce between 0.4 and 1.0 pounds TSS for each pound of BOD removed.

Solids production rates vary inversely with the SRT. Plants operating at low SRTs (higher food-to-microorganism (F/M) ratios) usually produce more solids per pound of BOD removed, while lower solids production rates correspond to high SRTs (lower F/M ratios).

Food-to-Microorganism Ratio
The F/M ratio should be used only as a secondary control measure to control sludge settleability. The F/M ratio is recommended as a guide to help the operator assess the process and adjust the SRT if the sludge settling characteristics deteriorate. Two separate F/M ratios can be considered: 

      · The ratio of the pounds of BOD5 in the wastewater in a 24-hour period to the pounds of MLVSS under aeration, 
         FB/M 
      · The ratio of pounds of ammonia nitrogen (NH4+-N) in the wastewater in a 24-hour period to the pounds of
        MLVSS under aeration. FN/M

During operation of the activated sludge system for nitrification, attention must be given to both F/M ratios. In general, the FB/M ratio will have the greatest effect on the physical operation of the system. For example, operation at either a too high or too low FB/M ratio will result in a poor settling sludge with excess solids being lost over the weirs of the secondary clarifiers. The FB/M ratio will also have a great effect on the population dynamics of the microorganisms contained in the activated sludge. Operation at a high FB/M ratio (generally above 0.2) will eventually wash out the slow-growing nitrifying bacteria, thus stopping nitrification.

The FN/M ratio can assist the operator in identifying nitrogen limiting conditions. Typical FN/M ratios are 1/4 to 1/5 of the FB/M ratio.

The general objective of the secondary system is to obtain a high F/M ratio in the anoxic selector to promote log growth phase conditions (active, rapid growth with the growth rate proportional to the existing biomass concentration) and maintain a low F/M ratio across the entire aeration basin so that the declining growth phase (net reduction in active biomass resulting from substrate depletion and resultant endogenous decay) occurs mainly in the aerobic zones.

The use of the step feed function of the aeration basin can significantly affect the F/M ratio in each zone. The F/M ratio can be maintained constant in each zone by adjusting the flow split.

RAS Pump Flow Rate
The RAS flow rate is more closely related to operation and performance of the secondary clarifiers and is discussed in more detail in the RAS Pumping section. It is briefly considered here to the extent that it affects the aeration, anoxic selection, and denitrification processes by returning oxidized nitrogen (nitrate and nitrite) to the aeration basins. Operation with an anoxic selector requires return of adequate amounts of nitrate to the head of the aeration basins. This can occur through the RAS, discussed below. Typical RAS rates vary between 25 and 75 percent of the influent flow. If the amount of nitrate returned is inadequate to maintain anoxic conditions, anaerobic selection mechanisms will be induced in the biomass in most cases. Anaerobic selection typically provides sludge settleability benefits equivalent to those resulting from anoxic selection.

Respiration Rate
The respiration rate is a measurement of the grams of oxygen consumed by 1 gram of MLVSS in a certain period of time (usually 1 hour). This calculation gives the operator an indirect measurement of the level of activity of the microorganisms in the activated sludge.

Respiration rate is calculated from the results of two tests: the oxygen uptake rate (OUR) and the MLVSS. The OUR is used to determine the rate of oxygen consumption (mg O2/l/hr) in an activated sludge sample.

The respiration rate of the activated sludge should be checked regularly and plotted on a trend chart with the other operational parameters that are being graphed by the operator. Generally, the lower the sludge age, the higher the respiration rate, and because of this relationship, respiration rates can be used to confirm the results of other tests and calculations that indicate the type or age of the activated sludge.

The respiration rate can be used in treatability studies to determine if a potential waste will be toxic to the activated sludge system and at what concentration. The respiration rate can also be used in toxicity studies to determine if a toxic substance has entered the activated sludge system. Normally, if a toxic substance has entered the activated sludge system, the respiration rate will be lower than normal.

Aeration Process DO Concentration
Dissolved oxygen concentration in the aeration basins is another important operating parameter of the secondary system. Aeration DO is controlled by controlling the total air flow rate from the process air blowers. The amount of air delivered by the aeration system is varied by selecting the number of blowers online and the position of the blower inlet throttling valves. The air flow to the basins can be automatically controlled by the PCS using a DO set point for the basins or by using an air flow rate (scfm) relative to plant influent flow.

Non-controllable Variables
Influent Flow Rate
The plant influent flow is dictated primarily by the flow rate into the Bar Screening from the Influent Pumping Station. This flow is weather dependent and will affect the hydraulic retention time, target solids retention time, mixed liquor suspended solids, F/M ratio, and solids production rate.

Influent Wastewater and Recycle Flow Characteristics
The influent concentrations of BOD, TSS, ammonia, phosphorus, etc., cannot be controlled, but will affect the solids retention time, mixed liquor suspended solids, F/M ratio, and solids production rate.

Calculations
Most of the equations presented in this section are derived from a material balance around the secondary system. A material balance is an accounting method for describing the movement of a particular material (e.g., BOD or TSS) through the system. The general word equation is:

      Mass added + Mass generated = Mass removed + Mass stored

The destruction of mass (such as BOD oxidation and conversion) is considered as a negative generation.
Material balances can be calculated for any material. They may be calculated just for the aeration basins or secondary clarifiers, or they may be calculated for the entire system. The material balance can provide much useful information concerning process performance. It can be used to assess the impact of sludge waste and sludge return flows, the generation or growth of activated sludge, and quantitative changes in the BOD and TSS throughout the plant.

Hydraulic Retention Time
The HRT is calculated as follows:

      HRT = VAB x 24/(QINF+QAB)
Where:
      · HRT = hydraulic detention time in hours
      · VAB = aeration basin(s) volume in million gallons
      · QINF = plant influent flow rate in million gallons per day (MGD)
      · QAB = RAS flow rate in MGD

Solids Retention Time
The solids retention time can be calculated by the following equation:



Where:
SRT = Solids retention time, days
VAB = Aeration basin volume, gallons
MLSS = Mixed liquor suspended solids, mg/L
NAB = Number of aeration basins on-line
QWAS = WAS flow, gallons per day (gpd)
XWAS = WAS suspended solids, mg/L
QE = Secondary clarifier effluent flow, gpd
XE = Secondary effluent TSS, mg/L

WAS Wasting Rate Based on SRT
Microorganisms in the aeration basin(s) use the organic material in the wastewater for energy and reproduction. The microorganisms produced, plus the inert and non-biodegradable solids that enter the system, must be wasted from the secondary treatment process to maintain the desired SRT.

The amount of sludge to be wasted is described by the SRT equation above. For a desired SRT and measured MLSS and XE, the WAS quantity to be wasted in one day can be determined by the following equation:



Where:
SRT = Solids retention time, days
VAB = Aeration basin volume, gallons
MLSS = Mixed liquor suspended solids, mg/L
NAB = Number of aeration basins on-line
QWAS = WAS flow, gpd
XWAS = WAS suspended solids, mg/L
QE = Secondary clarifier effluent flow, gpd
XE = Secondary effluent TSS, mg/L

An example of how to calculate the sludge waste rate by the SRT method with aeration basin 1 & 2 and secondary clarifier 1 &2 in service and a flow of 5.5 mgd is shown below.

Given:
   Aeration Basin Volume = 540,000 Gal
   Desired SRT = 5.5 Days
   Clarifier effluent TSS = 10 mg/L
   Aeration Basin MLSS = 3,100 mg/L
   Waste Sludge Concentration = 9,150mg/L
   Flow = 5,500,000 Gpd
   Number of aeration basins online = 2

Calculate the desired waste sludge amount

QWAS = 540000 gal ·3100 mg/L ·2 - 5500000 gal/day · 10 mg/L
                     5.5 day                                                       
                                       9150 mg/L

QWAS = 42 gal/min

Note: For operation of the sludge wasting, refer the WAS Pumping Section.

The sludge wasting procedures are very simple once the correct SRT has been chosen. However, the correct SRT varies with the wastewater, the aeration basin temperature, and the treatment process. Under this method, the aeration basins will seek the MLSS concentration needed to maintain the desired SRT.

Solids Production Rate
The equation for calculating the solids production rate is:



Where:
   Y = solids production (yield) rate, lb TSS produced/lb BOD removed 
   XE = Clarifier effluent TSS, mg/L 
   Q = average daily flow through secondary system, gpm 
   XWAS = WAS TSS, mg/L 
   QWAS = Sludge waste rate, gpm 
   MLSST = Today’s MLSS, mg/L 
   MLSST1 = Yesterdays MLSS, mg/L 
   VAB = Volume of aeration basin, gallons 
   NAB = Number of aeration basins on-line
   TBODI = Secondary influent total BOD, mg/L 
   SBODE = Secondary effluent soluble BOD, mg/L

An example of a solids production rate calculation with aeration basin 1 & 2 and secondary clarifier 1 & 2 in service and a plant flow of 5.5 mgd is as follows:
Given:
      XE =10 mg/L
      Q = 3820 gpm
      XWAS = 9150 mg/L
      QWAS = 158 gpm
      MLSST = 3111 mg/L
      MLSS T-1 = 3000 mg/L
      VAB = 540,000 Gallons
      TBODI = 84mg/L
      SBODE = 5 mg/L



Y = 0.85 Lb VSS/Lb BOD Removed


Food to Microorganism Ratio
The equations for the two F/M parameters are:



Where:
FN/M = ammonia nitrogen F/M ratio, d-1
BOD5 = 5-day carbonaceous BOD, mg/L
NNH4 = aeration basin influent ammonia-nitrogen, mg/L
Q = aeration basin influent flow, gpd
MLSS = mixed liquor suspended solids, mg/L
VAB = aeration basin volume, gallons


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Last Updated: 9/16/2013 9:17:24 AM
Version 4.0.1