Controlling Wastewater Treatment

1/Wastewater Plant Management

Success or failure in operating an activated sludge plant depends on proper control of the mass of active organisms in the plant. For example, variation of the solids retention time (SRT) will modify the entire process performance, including denitrification rates, sludge production and stability, mixed liquor concentration, oxygen uptake rates, and possibly the extent of nitrification. Increasing the SRT generally will increase oxygen requirements, the stability of the process, the extent of sludge stabilization, the extent of nitrification, and mixed liquor in concentration. Longer SRTs will decrease sludge production and denitrification rates in the post-aeration anoxic tanks. For pre-aeration anoxic tanks, longer SRTs increase the mixed liquor suspended solids (MLSS) and result in a greater denitrification rate.

2/Solids Control

The determinations of various forms of residue are useful in the control of a wastewater treatment plant. Total solids (TS), suspended solids (SS), dissolved solids (DS), and their volatile and fixed fractions, may be used to assess wastewater strength, process efficiency, and unit loadings. Measurements of the various residue concentrations are necessary to establish and assure satisfactory operation control.Total solids (TS), is a term applied to the weight of material per unit volume of a sample remaining after evaporation at a temperature of 103 - 105 degrees Centigrade. TS is equivalent to the sum of filterable and non-filterable residue. In wastewater, the term suspended solids (SS) corresponds to filterable residue. The term dissolved solids (DS) corresponds to the non-filterable residue. TS, DS, and SS may be further categorized as volatile or fixed (nonvolatile). Volatile solids are more or less equivalent to the fraction of residue that is organic, while nonvolatile solids approximate the inorganic fraction. Because solids determinations are empirical in nature, a consistent procedure should always be used and the procedure stated in the presentation of results.It has been estimated that, under normal operating conditions, about 1/3 of the incoming useable substrate is used for oxidation, while the remaining 2/3 is used for synthesis. Large portions of the incoming wastes are inert and not easily utilized. The result is that much of the substrate removed by the activated sludge floc remains in the floc and accumulates as either inert or living solids. Because of this collection and production of solids, the final settling tanks would eventually fill with solids, then flow over the effluent weir. Increasing the return sludge pumping rate without wasting some sludge would not solve the problem, because the sludge is just being moved around in the system and is not being removed.Ultimate control of the system, no matter what intermediate operating decisions are made, will always be based on solids wasting. There are three commonly used methods to decide how much sludge to waste: mixed liquor volatile suspended solids (MLVSS), food-to-microorganism (F:M) ratio, and sludge age.

3/Constant MLVSS

With this method, the operator is maintaining a constant mass of organisms to use the incoming food supply. Simply, if the operator finds that a mixed liquor volatile suspended solids (MLVSS) concentration of 2000 mg/L works effectively at the plant, that level will be maintained. If the solids in the aeration tanks increase above 2000 mg/L, the operator will waste more until the MLVSS level is again 2000 mg/L. If the MLVSS drops below 2000 mg/L, the operator will waste less and allow the solids concentration to increase.This system of solids control is simple to understand and manage, involves a minimum amount of lab work, and can produce good results. This method, however, has a rather severe limitation in that the important F:M ratio is ignored. Suppose, for example, that the biochemical oxygen demand (BOD) of the incoming waste increased by 50% over a substantial period of time. The increased solids production from the higher BOD load would be wasted to maintain the MLVSS level. The result, however, would be that the F:M ratio is 50% higher than the previously maintained ratio. The resulting high F:M or organic overload could easily lead to plant inefficiency or failure.

4/Constant F:M Ratio

Another method for control is wasting to maintain a constant food-to-microorganism (F:M) ratio. With this method, the operator will try to increase or decrease the MLVSS to match an increase or decrease in the BOD entering the plant. Most plants will operate best at a specific F:M ratio between 0.05 - 0.1. If the optimum F:M has been determined from experience and can be maintained, a good effluent may be produced with consistent plant operation.The F:M ratio is to be calculated at least weekly and related to the efficiency of treatment plant operation. An F:M ratio between 0.05 - 0.15 BOD/lb MLSS is usually considered acceptable for an extended aeration process. The food is calculated as the BOD in the influent that enters the aeration tank. If 175 mg/L BOD is the influent concentration at a flow rate of 3.0 MGD, the food available to the organism would be:

F = 175 mg/L x 3.0 MGD x 8.34 lb (mil gal)(mg/L) = 4375 lb/day

The mass of microorganisms is the average MLVSS concentration in the aeration tanks. For example, if the total aeration tank capacity were 2.06 mil gal, and the average MLVSS concentration in the aeration tanks 3000 mg/L, the estimated mass of organisms in the aeration tank would be:

M = 2.06 mil gal x 3000 mg/L x 8.34 lb (mil gal)(mg/L) = 51540 lb

The F:M ratio would then be:

4375 (lb BOD/day) / 51540 (lb MLVSS) = 0.8 lb BOD/day/lb MLVSS

This method of control would be most successful if the plant received a waste with very predictable variations; it requires a large amount of lab work because it is necessary to know both the amount of food added to, and the mass of, organisms in the system. Also, if the influent does not exhibit very predictable variations, the BOD or COD tests are too slow to allow MLVSS adjustments according to incoming waste strength.

5/Constant Sludge Age

The term sludge age is also commonly termed the sludge residence time (SRT), or mean cell residence time of the activated sludge system. Average sludge age will be simply the total amount of solids in the system divided by the amount leaving the system each day. Historically, sludge age has been calculated as a ratio of total solids (TS) in aeration to the weight of total solids (TS) in the aeration tank influent.

Sludge Age or Solids Retention Time or Mean Cell Residence Time, days


TS in aeration tank/solids wasted + solids lost in effluent per day

Therefore, if it takes 5 days to get rid of an amount of sludge equal to that contained in the system, then the sludge will be there 5 days, or have a sludge age of 5 days.

SRT = VaX/Qw Xu + Qe Xe

in which:

SRT = sludge age, days
Va = volume of aeration tanks, gal
X = average activated sludge concentration in the aeration tank, mg/L
Qw = flow rate of sludge being wasted, gpd
Xu = average concentration of activated sludge in final settling tank underflow,mg/L
Qe = flow rate of wastewater leaving plant, gpd
Xe = average solids concentration in the effluent, mg/L

The operator can set the sludge age (SRT) that has been found to work well for the particular plant. Therefore, to calculate the wasting rate (Qw), the operator needs to know only the volume of his aeration basin, the suspended solids (SS) concentration of the mixed liquor (X), and the return sludge (Xu). The only lab work involves determination of the mixed liquor and return sludge concentrations. The X:Xu ratio can be approximated very quickly by centrifuging samples of both the aeration mixed liquor and the return sludge and assessing the relative solids concentrations. If normal settling tests are done, the X:Xu ratio can also be approximated by taking the solids heights in a one L graduated cylinder after 30 minutes settling and dividing that height by the initial solids height in the cylinder. The overriding advantage of this method of solids control is that it is inherently simple and requires a minimum of lab work.

6/SRT(total) vs. SRT(oxic)

One operational parameter which deserves special mention is the oxic Solids Retention Time. Keeping a proper SRT(oxic) is important in the BioDenipho process so that efficient and reliable nitrification can be maintained. The SRT(oxic) is calculated by finding the process volume which is undergoing aeration (nitrification).For example, assuming the phase lengths are:

Phases A & D: 30 minutes (each)

Phases B & E: 60 minutes (each)

Phases C & F: 30 minutes (each)

*The oxic volume is 63% of the total. The aerated volume for the Water Reclamation Facility would equal:

(volume of the oxidation ditches) x (oxic volume fraction)

or

(1.32 MGD) x (0.63) = 0.83 MG

Therefore, in order to find the SRT(oxic), 0.83 MG would be the process volume. The SRT(oxic) would then be calculated as SRT(total) with only the applied volume being different:

SRT(oxic) / QwXu = X x V(oxic)