ACTIVATED SLUDGE

THE SCIENCE:
The activated sludge process involves blending settled primary effluent wastewater with a culture of microorganisms into a fluid called "mixed liquor". This mixed liquor is passed through an aeration tank which provides an adequate oxygen source for the type of activated sludge process(s) selected and to mix the "mixed liquor". The microbes eat (stabilize) the organic waste in the water. This conversion of biochemical oxygen demand (BOD) into a microbial culture obviously means that we will need to remove microbes from the process, in what we term waste activated sludge (WAS). The solids and microbes are separated from the wastewater being treated in a clarifier. A large portion of the microbes are returned to the aeration basin(s) as return activated sludge (RAS), to keep the cycle going.

The process mode design is selected to meet the process goal a) carbonaceous BOD removal and conversion and b) conversion of ammonia into nitrate (nitrification). In order to meet the goal, the hydraulic (liquid) detention time in the aeration basin(s), the microbes/solids detention time (solids retention time SRT or mean cell residence time MCRT) food to microorganism ration (F/M where the lbs of BOD/lbs of microbes under aeration), and the clarifier solids loading rate and hydraulic loading rate are determined. Ammonia, in higher concentrations, is harmful to the receiving waters, and may have to be reduced or eliminated. (Next year I will have a three part series on nitrification, and how it affects your facility, especially disinfection by chlorination).

Once the goal has been determined, the design team selects the Organic Loading Rate and then the aeration basin and secondary clarifier designs to achieve the goal.

TYPES (VARIATIONS) OF THE ACTIVATED SLUDGE PROCESS:

A) PLUG FLOW (CONVENTIONAL PLANT) Long narrow tank, or a series of several long tanks. Primary effluent and return activated sludge(RAS) combined at one end, with resulting lower dissolved oxygen (DO) at this point. There will be low C-BOD at the end of the last tank or pass. HAS NO REAL MIXER PER SAY, BUT THE BUGS AND FOOD ARE PASSIVELY MIXED. Not designed for nitrification, usually C-BOD removal only. Under-loaded process may successfully operate in the nitrification mode.

B) COMPLETE MIX IS WHERE THE MICROORGANISMS AND THE WASTE STREAM ARE MIXED TOGETHER IN SUCH A MANNER TO INSURE A COMPLETE MIX OF THE TWO COMPONENTS. WITH CAREFUL ATTENTION TO THE DESIGN OF THE TANK, AND FOLLOW-UP ON THE mixed liquor suspended solids (MLSS) AND D.O. IN THE AERATION BASIN, IT SHOULD BE UNIFORM. MLSS IS FROM 2000 to 5000 mg/l. Aeration basins are usually circular or square. Best variation of the activated processes to handle slug loads. The major problem seems to be a greater tendency to grow filamentous microbes which causes "bulking" (discussed in part three.)

C) STEP FEED (STEP AERATION) ALLOWS FOR THE FOOD TO BE INTRODUCED TO THE MICROBIAL POPULATION AT SUCCESSIVE POINTS IN THE AERATION TANK(S). Allows for better DO control, and F/M control. Reduces the oxygen demand at the inlet end of the tank where the return activated sludge and the primary effluent join and enter the aeration tank, like in a conventional process.

D) CONTACT STABILIZATION (sludge re-aeration) IS WHERE THE BUGS ARE RE-AERATED AFTER THE CLARIFIER AND BEFORE BEING INTRODUCED INTO THE AERATOR WITH THE FOOD. THIS ALLOWS THEM TO BRING INTO THEIR CELLS THE FOOD THAT IS ATTACHED TO THE OUTSIDE OF THEIR CELL WALLS. MLSS IS FROM 1500 to 2000 mg/l. This variation reduces the require aeration basin capacity and clarifier solids loading rates.

E) EXTENDED AERATION MEANS THAT THE MCRT IS QUITE HIGH. MLSS ARE FROM 2000 to 5000 mg/l. DUE TO THE LOW FOOD/HIGH MICROBE RATIO (F/M RATIO), EVEN THE STORED FOOD IN THE DEAD MICROORGANISMS IS CONSUMED, AND THEREFORE DOES NOT PRODUCE AS MUCH waste activated sludge (WAS) AS OTHER PROCESSES. OXIDATION DITCHES are extended aeration mode variants, WHICH USE BRUSHES TO AERATE THE BUGS AND FOOD AS THEY MOVE ABOUT IN A CIRCULAR RACE TRACK. Most all nitrify.

F) TAPERED AERATION USES MORE AIR AT THE ENTRY POINTS OF THE AERATION TANK WHERE THE DO DEMAND IS AT IT’S HIGHEST VALUES THAN AT THE EXIT, WHERE THE OXYGEN DEMAND IS LESS. The attempt is to address DO problems in the conventional (plug flow0 variation. Tanks are usually long and narrow, and in series.

G) PURE OXYGEN PLANTS USE, AND MAY EVEN GENERATE THEIR OWN, PURE OXYGEN TO USE IN THEIR AERATION PROCESS. The aeration tanks are covered. Allows for a greater organic loading (C-BOD)per 1000 cubic block of aeration capacity.

H) KRAUS PROCESS USES DIGESTER SUPERNATANT TO CORRECT NITROGEN DEFICIENCIES IN THE WASTEWATER STREAM TO BE TREATED. Generally not considered a true variation or modification of the activated sludge process.

NITRIFICATION: depending on the temperature of the water, the BOD loading, etc., a mean cell residence time (MCRT) is selected to cultivate a microbial population that will create and maintain a population of nitrifying bacteria.

Some selected DESIGN PARAMETERS FOR ACTIVATED SLUDGE:

Food/microorganism ratio (F/M) for Conventional Activated Sludge 0.2 to 0.4
Food/microorganism ratio (F/M) for extended aeration less than 0.05
Food/microorganism ratio (F/M) for nitrification 0.05 to 0.15

Dissolved Oxygen (DO): bulking potential less than 1 mg/l
Dissolved Oxygen (DO) normal 1 to 2 mg/l
Dissolved Oxygen for "Nitrification" is 2 to 3 mg/l

Sludge Volume Index (SVI) values: pinfloc potential less than 50 ml/g
Sludge Volume Index (SVI) values: good range 50 to 100 ml/g
Sludge Volume Index (SVI) values: Filament growth 100 to 150 ml/g
Sludge Volume Index (SVI) values: Bulking at high flows 150 to 200 ml/g
Sludge Volume Index (SVI) values: Bulking 200 to 300 ml/g
Sludge Volume Index (SVI) values: Severe bulking higher than 300
These SVI values are only approximate, as each clarifier and the upstream aeration basin loading and type will determine YOUR values.

Mean Cell Residence Time (MCRT) with NO nitrification 1 to 3 days
Mean Cell Residence Time (MCRT) nitrification potential when it is 4 to 7 days
Mean Cell Residence Time (MCRT) complete nitrification 8 to 12 days
High oxygen 12 to 20 days
Extended aeration more than 20 days
MCRT means the average microbe’s life span in the system, before being wasted away or lost over the secondary clarifier weirs.

BOD conversion conventional 0.4 to 0.8 lb solids/lb BOD removed
Extended aeration 0.1 to 0.3 lb solid/lb BOD removed

Oxygen Consumption: conventional 0.5 to 1.1 lbs O2/lb BOD removed
Extended aeration 1.2 to 1.6 lb O2/lb BOD removed

"The Operation of the Process"
The activated sludge process is regarded by most operators to be the most difficult of all of the wastewater biological secondary treatment processes. But if the process has been properly designed, and a has an adequate laboratory support, properly trained operators rarely have major problems with the process.

There are some excellent texts covering activated sludge, such as the chapters in Water Environment’s MOP 11, and No. OM-9, "Activated Sludge". I will certainly not try to write such volumes, as these two texts, of my personal experiences for this column! Rather, it is my desire to concentrate on some operational items which are not generally discussed a lot at this time: selectors, primary design and operating criteria that are important to operate the process well, and optimizing the process in your facility.

First some definitions:
The solids retention time (SRT) is usually defined as the number of pounds of solids under aeration divided by the sum of the number of pounds activated sludge wasted added to the number of pounds of solids lost in the secondary effluent.

The mean cell residence time (MCRT) is most often defined as the number of pounds of solids in the aeration basin added to the number of pounds of solids in the secondary clarifier divided by the sum of the number of pounds of activated sludge wasted added to the number of pounds of solids lost in the secondary clarifier effluent. We sample the solids in the secondary clarifiers using sludge judges to increase accuracy. I personally feel that this is the best approach.

Selectors are small basins in which the mixed liquor (mixture of Return Activated Sludge [RAS] and primary effluent) is treated (passed through) prior to the activated sludge process. The basins provide about 15 to 30 minutes of hydraulic detention time. The selectors are designed for aerobic, anaerobic, or anoxic conditions, depending on the process goal.:
Aerobic selectors require 2 to 3 mg/L dissolved oxygen concentration to reduce the filamentous bacteria growth potential

Anaerobic selectors have a very low DO content (< 0.5 mg/L DO) with low concentrations of nitrates from the secondary clarifier (< 5 mg/L) to also reduce filamentous microbes. This occurs due to the low dissolved oxygen concentrations, and a high concentration of a phosphorus consuming population of microbes that consume a majority of the CBOD in the selector which makes the food unavailable to the filamentous microorganisms later in the aeration basin(s).

Anoxic selectors are created when a recycled flow of treated secondary effluent, containing high amounts of nitrates, is returned to the selector to mix with the mixed liquor. This selector is used to denitrify (remove a portion of the nitrate-nitrogen in the effluent).

(In Vacaville I have created an anaerobic selector in my conventional activated sludge aeration basins by reducing the quantity of air that is applied at the beginning of the first aeration basin, thereby lowering the dissolved oxygen concentration. The amount of air is very low, only enough to satisfy mixing requirements. I then apply enough air to maintain 2 to 3 mg/L DO throughout the rest of the basins. Profile tests have shown the release and subsequent uptake of phosphorus, a primary characteristic of anaerobic selector. Eight years of personal experience with this anaerobic selector in my own plant has shown that the selector allows for the phosphorus group of microorganisms to consume the carbonaceous biochemical oxygen demand (CBOD) in the selector, thereby reducing the level of food quantity available for filamentous growth and other microbes later in the process.)

The primary design criteria for all activated sludge process variations include:
a. the selected food (CBOD) to microorganism (F/M) ratio and the corresponding solids retention time (SRT) or means cell resident time (MCRT)
b. the hydraulic detention time of the wastewater being treated in the aeration basins
c. aeration blower capacity, oxygen uptake rate of the mixed liquor, and type of diffuser
d. the secondary clarifier design criteria…hydraulic, and solids loading rates
e. waste activated sludge (WAS) and return activated sludge (RAS) design parameters
f. wastewater treatment stream characteristics

Operational controls: there are several
1) Return activated sludge modes of operation include:
a) Constant flow rate: the return activated sludge is pumped from the secondary clarifier(s) back to the aeration basin(s) at a constant flow rate disregarding all other process variables.
b) Raw wastewater flow paced: the return activated sludge is returned according to a rate to that is proportional to the incoming wastewater to be treated. Usually this means that as the wastewater flow increases, the flow rate of the return activated sludge is increased on a preset proportional rate.
c) Combination flow rates: some facilities utilize both constant flow rates and flow proportional flow rates depending on the season, wastewater temperature, changes in organic loading rates, and also time of day.

2) It is very important to maintain the selected quantity of microorganisms in your aeration basin(s), and/or clarifier(s) (SRT or MCRT). This is accomplished daily by removing from the system (wasting) each days growth of microorganisms, in what we call waste activated sludge. The wasting of the excess microbes from your process is the most important process variable, as it affects all of the other variables.

Wasting of activated sludge is basically one of two methods:
a) Constant flow rate: the wasting rate of the activated sludge is constant over the entire day, based upon the number of dry pounds of activated sludge that need to be wasted to hold a specific MCRT or SRT.
b) Batch wasting rate: treatment processes that have high MCRTs or SRTs, such as extended aeration plants, may waste solids from their system in small batches daily, or once per day. This is usually not recommended, but has provided satisfactory results for many small facilities with such processes as extended aeration - oxidation ditch.

3) Dissolved oxygen concentration
The microorganisms require dissolved oxygen in the water in order to carry out their metabolic processes, and to stabilize the organics (CBOD) in the wastewater. There must be sufficient dissolved oxygen not only for the free swimming and small colonies of microbes, but also for those microbes that are in the center of larger flock particles.
Most processes, without selector's, hold 2 to 3 mg/L dissolved oxygen concentration. Above 3 mg/L DO concentration electrical power is usually wasted. Filamentous microorganisms are usually able to out compete more desirable microbes when the DO concentration is below 1mg/L. (Filamentous microorganisms in higher proportions to other microbes can cause poor sludge settling problems in the clarifier's.) As stated previously, processes with selectors will have dissolved oxygen concentrations that will vary from these values.
Additional oxygen, above that required for carbonaceous biochemical oxygen demand (CBOD) removal, is required if the treatment process is to convert ammonia into nitrates (nitrification).

Optimizing your activated sludge process:
Using the X axis (horizontal) for the time value of days, graph (trend) your treatment plants influent and effluent BOD and suspended solids on the Y axis (vertical). Using the same scale of the X axis, graph the corresponding activated sludge process values of F/M ratio, MLSS or MLVSS, SRT or MCRT, SVI and DO concentrations. The goal of this task is to correlate process parameters to the best quality of plant effluent. Once the best quality of effluent is found, the process parameters that achieved this water quality are selected for operational use. Remember, in our smaller communities we generally have greater variations in flow rates, suspended solids, and CBOD loadings on the plant. Our challenge therefore, is to hold our proportional process variables as stable as we can, while the loadings vary somewhat. (Those of us who have the "thrill" of operating resorts, weekend community facilities, etc., have learned this skill all too well!) We can best do this by selecting the best SRT or MCRT value that the graphs indicate and apply sufficient dissolved oxygen to meet the corresponding oxygen demand. We calculate the SRT or MCRT at least daily. We then set our wasting rate of activated sludge on a five-day or seven-day running the average. This helps to stabilize swings in the number of pounds of microbes in our systems. Generally speaking, for every pound of BOD entering the process we create approximately 0.4 to 0.6 pounds of microorganisms. The microbes must therefore be wasted from the system, or soon the clarifiers will be filled with solids!

Generally speaking, even small changes in the operating parameters such as the MCRT or SRT, F/M ratio, and mixed liquor suspended solids concentration require as much as two or three times the MCRT or SRT value to stabilize to the new parameters. Patience is the key here!

Process parameters and Laboratory sampling and analysis
The type of tests and their frequency for laboratory sampling and analysis will be dictated by the magnitude of the variations in the wastewater flow stream and the CBOD concentrations entering the treatment plant, the design of the activated sludge process. The following should be considered for control of an activated sludge process:

aeration basin:
dissolved oxygen concentration
mixed liquor suspended solids
mixed liquor volatile suspended solids
sludge volume index (SVI)

Secondary clarifier:
sludge blanket depths and suspended solids concentration in the clarifier

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