Fixed film and activated sludge — the operating parameters and what they look like when they are working. Built on the New Mexico Operator Certification Study Manual, Chapters 7 and 10.
Only after the F:M is understood can the other operating factors be assessed.
Source: NMED / NMSU WUTAP Operator Certification Study Manual, January 2005
In order to remove the very small suspended solids (colloids) and dissolved solids, wastewater treatment plants include Secondary Treatment. This process produces an overall plant removal of suspended solids and BOD of 90% or more.
— Chapter 7
| Item | Detail |
|---|---|
| The target | COLLOIDS and DISSOLVED SOLIDS — the fraction primary clarification cannot settle. |
| Performance | 90% OR MORE overall plant removal of suspended solids and BOD. |
| The three common processes | TRICKLING FILTERS, ROTATING BIOLOGICAL CONTACTORS, and ACTIVATED SLUDGE. |
| The two families | FIXED FILM (Chapter 7) and SUSPENDED GROWTH (Chapter 10). |
| Activated sludge capability | "BOD and TSS removal rates IN EXCESS OF 99% ARE NOT UNUSUAL." |
FIXED FILM: the organisms grow as a slime layer ON MEDIA that stays put, and the wastewater passes over them.
SUSPENDED GROWTH: the organisms are held IN SUSPENSION in the wastewater itself, and are separated afterward by settling.
That single difference drives everything else — what the operator controls, what goes wrong, and how much attention the process needs.
The manual is direct about the tradeoff: "IN ACTUAL OPERATION, THE TRICKLING FILTER IS ONE OF THE MOST TROUBLE-FREE TYPES OF SECONDARY TREATMENT. THIS PROCESS REQUIRES LESS OPERATING ATTENTION AND CONTROL THAN OTHER TYPES."
The term "filter" is rather misleading because it indicates that solids are separated from the liquid by a straining action. This is not the case.
— Chapter 7
What actually happens: passage of wastewater through the filter develops a GELATINOUS COATING of bacteria, protozoa and other organisms on the media — the ZOOGLEAL FILM. That growth absorbs and uses the suspended, colloidal and dissolved organic matter as the wastewater passes over it in a thin film. Part of the material becomes new cells; another portion is OXIDIZED TO CARBON DIOXIDE AND WATER. Excess and dead film is continuously or periodically SLOUGHED off and leaves with the effluent.
| Part | Detail |
|---|---|
| THE MEDIA | Provides a large surface area for the slime growth. Rock, slag, coal, bricks, redwood blocks, molded plastic or any sound durable material. ROCK IS USUALLY 2–4 INCHES. "Actual size is not too critical, but IT IS IMPORTANT THAT THE MEDIA BE UNIFORM IN SIZE TO PERMIT ADEQUATE VENTILATION." |
| THE UNDERDRAIN SYSTEM | A sloping bottom leading to a center channel that collects filter effluent. Spaced redwood stringers or prefabricated blocks of concrete, vitrified clay or other suitable material. |
| THE DISTRIBUTION SYSTEM | Usually a ROTARY DISTRIBUTOR — horizontal pipes on a central column, gravity fed, distributing through ORIFICES ALONG ONE SIDE of each arm. ROTATION IS DUE TO THE WATER-SPRINKLER REACTION. |
| Media | Depth | VOID SPACE |
|---|---|---|
| Rock | 3–8 feet | ABOUT 35% |
| Synthetic (plastic) | 15–30 feet | ABOUT 95% |
Rock gives about 35% void space. Plastic gives about 95% — nearly three times the room for air to ventilate and for slimes to slough and pass through.
The manual states the consequence directly: "TRICKLING FILTERS WITH PLASTIC MEDIA MAY BE LOADED AT MUCH HIGHER RATES THAN ROCK MEDIA WITHOUT DEVELOPING PLUGGING, PONDING, AND FLY OR ODOR PROBLEMS."
That is why plastic-media towers run 15–30 feet deep while rock filters stay at 3–8 feet. The depth is only possible because the voids stay open.
And the reason uniform media size matters: in nonuniform media, THE SMALLER PIECES FIT BETWEEN THE LARGER ONES and make it easier for slimes to plug the filter.
The biological film requires A CONTINUOUS SUPPLY OF DISSOLVED OXYGEN adsorbed from air circulating through the filter voids. The manual: ADEQUATE VENTILATION OF THE FILTER MUST BE PROVIDED; THEREFORE THE VOIDS IN THE FILTER MEDIA MUST BE KEPT OPEN. Ventilation may be natural or forced air. Clogged void space creates PONDING AND REDUCTION IN OVERALL FILTER EFFICIENCY.
| What it does | Detail |
|---|---|
| Contact and seeding | Filter effluent is recycled and contacts the film MORE THAN ONCE, and helps SEED THE LOWER PORTIONS of the filter with active organisms. |
| Induces sloughing | Higher flows per unit area cause MORE CONTINUOUS AND UNIFORM SLOUGHING of excess or aged growths, which PREVENTS PONDING AND IMPROVES VENTILATION. |
| Suppresses pests | Increased hydraulic loadings DECREASE THE OPPORTUNITY FOR SNAIL AND FILTER FLY BREEDING. |
| Dilutes strength | Growth thickness is DIRECTLY RELATED TO ORGANIC STRENGTH — THE HIGHER THE BOD, THE THICKER THE LAYERS. Recirculation dilutes it. |
| Keeps things moving | Sometimes practiced only during low flow to keep rotary distributors turning, prevent drying of growths, or PREVENT FREEZING. |
The manual describes a practice worth knowing: "SOME PLANTS OPERATE INTERMITTENTLY AT HIGH RECIRCULATION RATES (ALL RECIRCULATION PUMPS ON) FOR TWO OR THREE HOURS EACH WEEK."
The purpose: "THIS HIGH RATE WILL CAUSE SLOUGHING ON A REGULAR BASIS RATHER THAN ALLOWING THE SLIME GROWTHS TO BUILD UP AND SLOUGH UNDER UNCONTROLLED CONDITIONS."
That converts an unpredictable event that upsets the clarifier into a scheduled one you can staff for. It also controls filter flies — the same weekly flush appears in the fly control list.
| Class | Hydraulic loading (gal/day/sq ft) | Organic loading (lbs BOD/day/1,000 cu ft) | Media and depth |
|---|---|---|---|
| STANDARD-RATE | 25 to 100 | 5 to 25 | Usually rock, 6–8 feet |
| HIGH-RATE, rock | 100 to 1,000 | 25 to 100 | Rock or synthetic, 3–5 feet |
| HIGH-RATE, synthetic | 350 to 2,100 | 50 to 300 | — |
| ROUGHING | — | 100 to over 300 | A high-rate filter at very high organic loading |
Hydraulic loading is total volume INCLUDING RECIRCULATION, per square foot of filter surface area. Where recirculation is used an additional organic loading is placed on the filter, but this added loading IS OMITTED IN MOST CALCULATIONS because it was included in the influent load.
| Class | Character |
|---|---|
| STANDARD-RATE | Growth often heavy — in addition to bacteria and protozoa, MANY TYPES OF WORMS, SNAILS AND INSECT LARVAE. Sloughs at intervals, NOTICEABLY IN SPRING AND FALL. Effluent usually QUITE STABLE with BODs AS LOW AS 20 TO 25 mg/L. |
| HIGH-RATE | Receives wastewater continually; PRACTICALLY ALL HIGH-RATE INSTALLATIONS USE RECIRCULATION. More uniform sloughing, but the material is LIGHTER AND MORE DIFFICULT TO SETTLE. |
| ROUGHING | Reduces THE ORGANIC LOAD ON SUBSEQUENT PROCESSES — a second-stage filter or activated sludge. Common with STRONG INDUSTRIAL WASTES, or where 50–70% BOD REMOVAL IS SATISFACTORY. |
| Cause | Detail |
|---|---|
| Excessive organic loading | Without a corresponding high recirculation rate. |
| THE MOST COMMON SOURCE | "PERHAPS THE MOST COMMON SOURCE OF PONDING IS FROM THE LACK OF GOOD PRIMARY CLARIFICATION PRIOR TO THE FILTER." |
| Media too small or nonuniform | Smaller pieces fit between larger ones. IF THIS CONDITION EXISTS, REPLACEMENT OF THE MEDIA IS THE MOST SATISFACTORY SOLUTION. |
| Poor media cementing or breaking up | — |
| Fibers or trash in the voids | — |
| Shock load after high growth | A high organic growth rate followed by a shock load and RAPID UNCONTROLLED SLOUGHING. |
| Insect larvae or snails | Excessive growth accumulating in the voids. |
Spray the filter surface with a HIGH PRESSURE WATER STREAM.
HAND TURN OR STIR the filter surface with a rake, fork or bar. Remove accumulated leaves or debris.
DOSE THE FILTER WITH CHLORINE AT ABOUT 5 mg/L FOR SEVERAL HOURS.
FLOOD THE FILTER, keeping the media submerged for 24 HOURS — this will cause the growth to slough.
SHUT OFF FLOW to the filter for several hours, ALLOWING THE GROWTH TO DRY OUT.
The manual is explicit: "THE CAUSE OF PONDING SHOULD BE IDENTIFIED AS IT WILL AFFECT THE CORRECTIVE STEPS TAKEN TO SOLVE THE PROBLEM."
Flooding a filter that is ponding because the media is the wrong size will buy you a few days. Fixing the primary clarifier upstream fixes it permanently.
Note that two of the five remedies — flooding and drying — work by killing the growth. They are resets, not adjustments.
Since operation of trickling filters is an aerobic process, no serious odors should exist unless odor producing compounds are present in the wastewater in high concentrations. The presence of foul odors indicates that anaerobic conditions are predominant.
— Chapter 7
The manual notes that anaerobic conditions ARE usually present under the portion of slime growth NEXT TO THE MEDIA SURFACE — that is normal. AS LONG AS THE SURFACE OF THE ZOOGLEAL FILM IS AEROBIC, ODORS SHOULD BE MINOR.
| Corrective guideline | Detail |
|---|---|
| 1 | MAINTAIN AEROBIC CONDITIONS IN THE SEWER COLLECTION SYSTEM AND IN THE PRIMARY TREATMENT UNITS. |
| 2 | CHECK VENTILATION IN THE FILTER. Heavy biological growths or obstructions in the underdrain system will cut down ventilation. |
| 3 | INCREASE THE RECIRCULATION RATE to provide more oxygen to the bed and increase sloughing. |
| 4 | Keep the wastewater splash from the distributors IN the filter and AWAY from exposed structures, grass and other surfaces. |
| Item | Detail |
|---|---|
| The insect | The tiny, gnat-sized FILTER FLY (PSYCHODA). |
| Where they thrive | Preferring an ALTERNATELY WET AND DRY environment, they are found MOST FREQUENTLY IN LOW RATE FILTERS and are usually NOT MUCH OF A PROBLEM IN HIGH RATE FILTERS. |
| THE LIFE CYCLE | "AS SHORT AS SEVEN DAYS" — which sets the control interval. |
| Control 1 | Increase recirculation rate. Synthetic media require higher hydraulic loadings or WEEKLY FLUSHING by turning on all the filter pumps. |
| Control 2 | Keep orifice openings clear, INCLUDING END GATES of distributor arms. |
| Control 3 | Apply approved insecticides WITH CAUTION to filter walls and other plant structures. |
| Control 4 | FLOOD THE FILTER FOR 24 HOURS at intervals frequent enough to PREVENT COMPLETION OF THE INSECT LIFE CYCLE. |
| Control 5 | Shrubbery, weeds and tall grass provide A NATURAL SANCTUARY. Good housekeeping and grounds maintenance minimize fly problems. |
Every effective control on that list runs on a WEEKLY interval — weekly flushing, flooding at intervals frequent enough to break the cycle.
That is not a coincidence. If the life cycle is as short as seven days, an intervention every two weeks lets a full generation complete.
It also lines up with the weekly high-recirculation sloughing practice in Part 2 — one operating routine addresses both.
The manual gives a genuinely elegant diagnostic: "MEASURE BOTH THE SOLUBLE AND TOTAL BOD IN THE FINAL EFFLUENT."
HIGH TOTAL BOD → the poor effluent is caused by BOD ASSOCIATED WITH ESCAPING SOLIDS. That is a settling and sloughing problem.
HIGH SOLUBLE BOD → THE TRICKLING FILTER BOD REMOVAL CAPACITY IS BEING EXCEEDED. That is a loading problem.
Two very different responses, distinguished by one extra lab test.
| Measure | Detail |
|---|---|
| 1 | DECREASE THE AMOUNT OF RECIRCULATION, provided sufficient flow remains to keep the filter working. |
| 2 | Operate TWO STAGE FILTERS IN PARALLEL rather than in series. |
| 3 | Adjust or remove ORIFICES AND SPLASH PLATES to reduce the spray effect. |
| 4 | Construct WIND SCREENS, COVERS OR CANOPIES to reduce heat loss. |
| 5 | Physically break up and remove larger areas of ice buildup. |
The manual: although filter efficiency is reduced during icing, IT IS IMPORTANT TO KEEP THIS UNIT RUNNING.
Taking it out of service NOT ONLY REDUCES EFFLUENT QUALITY BUT MAY LEAD TO ADDITIONAL MAINTENANCE PROBLEMS, SUCH AS ICE FORMING, WITH THE POSSIBILITY OF STRUCTURAL DAMAGE.
And a detail easy to miss: "MOISTURE MAY CONDENSE IN THE OIL AND DAMAGE THE BEARINGS."
| Item | Detail |
|---|---|
| Location | Bearings may be in the BASE of the center column or at the TOP. Both have a water seal at the base to prevent leakage — to avoid uneven distribution AND to protect base-mounted bearings. |
| MERCURY SEALS | "Many older distributors used a mercury seal. MERCURY SHOULD NOT BE USED BECAUSE MERCURY IS TOXIC TO LIVING ORGANISMS, INCLUDING OPERATORS." |
| Bearings | Ride on REMOVABLE RACES (tracks) in a bath of oil. Usually TURBINE OIL with oxidation and corrosion inhibitors. |
| THE OIL CHECK | WATER IN THE OIL WILL APPEAR AT THE BOTTOM OF THE OIL IN THE CONTAINER. If water is found, EITHER THE SEALING FLUID IS LOW OR THE GASKET MUST BE REPLACED in mechanical seals. |
| Task | Detail |
|---|---|
| SAFETY FIRST | "WORK ON DISTRIBUTOR ORIFICES ONLY AFTER THE ARMS HAVE STOPPED MOVING." |
| Weekly | FLUSH THE ARMS by opening the end dump gates ONE AT A TIME. |
| Daily | Clean debris off the filter surface. Clean orifices as often as needed. OBSERVE THE DISTRIBUTOR DAILY for smooth operation. |
| THE FAILURE SIGNAL | "If it becomes JUMPY, seems to VIBRATE, or SLOWS DOWN WITH THE SAME AMOUNT OF WASTEWATER PASSING THROUGH IT, THE BEARINGS AND RACES ARE PROBABLY DAMAGED and will require replacement." |
| Leveling | Adjust the turnbuckles occasionally on the guy rods to keep the arms at the proper level for even flow. |
| NORMAL SPEED | On larger distributors, APPROXIMATELY 1 RPM IS NORMAL. |
| Too fast | "IF THE DISTRIBUTOR ROTATES TOO FAST, IT MAY DAMAGE THE BEARING RACES ON THE TURNTABLE." |
| Speed control | Provision is usually made on the FRONT of each arm for orifices — the reaction of water through these CANCELS SOME OF THE THRUST of the regular orifices. |
The manual: "SINCE MOST DISTRIBUTORS APPEAR RATHER LARGE AND BULKY, MANY OPERATORS ARE SURPRISED TO FIND THAT THEY ARE DELICATELY BALANCED. AS SOON AS WASTEWATER BEGINS TO FLOW FROM THE ORIFICES, THE DISTRIBUTOR ARM SHOULD START TO MOVE."
That is also a diagnostic. A distributor that needs a push to start, or that will not turn at low flow, has a bearing or a leveling problem — not a flow problem.
Activated sludge is a suspended growth secondary treatment process that primarily removes dissolved organic solids as well as settleable and non-settleable suspended solids.
— Chapter 10
The term "ACTIVATED" comes from the fact that THE PARTICLES ARE TEEMING WITH BACTERIA, FUNGI AND PROTOZOA. Organisms are cultivated in aeration tanks where they are provided with dissolved oxygen and food from the wastewater.
| Step | What happens |
|---|---|
| 1 | Wastewater enters the aeration basin. THE MICROBES CONSUME THE SOLIDS. |
| 2 | The mixture of wastewater and microorganisms is MIXED LIQUOR. |
| 3 | In the secondary clarifier, activity slows and organisms CLUMP TOGETHER — BIO-FLOCCULATION, "the ability of one floc particle to stick to another." |
| 4 | Because clarifier velocity is very low, the flocculated clumps SETTLE as sludge while clarified water flows over a weir. |
| 5 | Settled organisms are pumped back to the front of the aeration basin — RETURN ACTIVATED SLUDGE (RAS). |
| 6 | Excess organisms are removed — WASTE ACTIVATED SLUDGE (WAS). |
The manual: "MICROORGANISMS CAPTURE MUCH OF THE DISSOLVED ORGANIC SOLIDS IN THE MIXED LIQUOR RAPIDLY (MINUTES), HOWEVER, MOST ORGANISMS WILL REQUIRE A LONG TIME TO METABOLIZE THE FOOD (HOURS)."
That is why an aeration basin needs hours of detention for a process that captures in minutes — and it is exactly the principle Contact Stabilization exploits by splitting the two functions into separate tanks.
| Control | What it does |
|---|---|
| PROVIDING CONTROLLABLE INFLUENT FEEDING | Ensures even loading to all aeration basins. Flow splitter boxes should be CHECKED PERIODICALLY to ensure the split is as intended. |
| MAINTAINING PROPER DISSOLVED OXYGEN AND MIXING | A minimum of 1.0 mg/L D.O. is recommended for most basic types. |
| CONTROLLING THE RAS PUMPING RATE | Determines how long solids sit in the clarifier. |
| MAINTAINING THE PROPER MIXED LIQUOR CONCENTRATION | This IS controlling the F:M ratio of the system. |
| Point | Detail |
|---|---|
| What it is | Feeding wastewater THROUGHOUT VARIOUS POINTS in the aeration basin rather than only at the head. |
| The benefit | RELIEVES THE HIGH OXYGEN DEMAND that occurs where the influent flow and RAS enter the basin. |
| THE DOWNSIDE | "Some of the dissolved solids in the influent MAY PASS THROUGH THE AERATION BASIN TOO RAPIDLY, AND SHOW UP IN THE EFFLUENT AS BOD." |
| Condition | Consequence |
|---|---|
| TOO LOW, for long periods | UNDESIRABLE ORGANISMS, SUCH AS FILAMENTOUS TYPE BACTERIA, MAY DEVELOP AND OVERTAKE THE PROCESS. |
| Insufficient generally | Slows down or kills aerobic organisms, makes facultative organisms less efficient, and leads to FOUL-SMELLING BY-PRODUCTS OF ANAEROBIC DECOMPOSITION. |
| TOO HIGH | Can cause FLOC PARTICLES BEING FLOATED TO THE SURFACE of the secondary clarifiers — "PARTICULARLY COMMON DURING COLD WEATHER." |
| The recommendation | A minimum of 1.0 mg/L in the aeration tank. |
"IT IS IMPORTANT TO UNDERSTAND THAT THE REQUIRED LEVEL OF DISSOLVED OXYGEN IS ACTUALLY RELATED TO THE F:M RATIO THAT THE SYSTEM IS OPERATING UNDER."
And then the practical consequence: "SOME SYSTEMS RUN AT LESS THAN 1.0 mg/L OF D.O. AND YET OPERATE WELL BECAUSE THEY ARE STILL OPERATING WITHIN AN ACCEPTABLE F:M RANGE. SOME HIGHLY LOADED SYSTEMS NEED MUCH MORE THAN 1.0 mg/L JUST TO GET BY."
So 1.0 mg/L is a starting point, not a specification. The F:M tells you what the number should actually be — which is why the manual insists F:M is determined first.
A temperature note worth having: IT IS EASIER TO DISSOLVE OXYGEN INTO COLD WATER THAN WARM. Cold weather INCREASES aeration system performance, although microorganism activity is reduced.
| Parameter | CONVENTIONAL | EXTENDED AERATION | CONTACT STABILIZATION |
|---|---|---|---|
| Aeration detention | 4–8 hrs | 12–24 hrs | Contact 0.3–3 hrs; stabilization 4–8 hrs |
| MLSS | 1,000–4,000 mg/L | 2,000–5,000 mg/L | Contact 1,000–3,000 mg/L; stabilization 2–6× contact |
| SRT | 3.5–10.0 days | GREATER THAN 10 days | LESS THAN 3.5 days |
| F:M ratio | 0.25–0.5 : 1 | 0.05–0.15 : 1 | 0.5 to over 1.0 : 1 |
| RAS pumping | 15–75% of influent | 50–150% of influent | 25–100% of influent |
Source: Chapter 10. The manual notes general RAS rates of between 25% and 150% of influent flow are commonly used.
The primary difference between these three modes of operation has to do with the length of time that the microorganisms reside in the treatment system.
— Chapter 10
SRT = lbs MLSS in the system ÷ lbs suspended solids entering per day
Example from the manual:
1,000 lbs under aeration ÷ 100 lbs/day entering = SRT of 10 days
| Mode | Character |
|---|---|
| CONVENTIONAL | THE MOST COMMON TYPE IN USE TODAY. Usually equipped with primary clarification. Produces a high quality effluent and CAN ABSORB SMALL SHOCK LOADS without lowering effluent quality. |
| EXTENDED AERATION | Often used in SMALLER PACKAGE-TYPE PLANTS and complete oxidation systems. VERY STABLE due to light loading. Low F:M made possible by LARGER AERATION BASINS and sludge ages over 10 days. BUT: "IT IS COMMON FOR EXTENDED AERATION SYSTEMS TO DISCHARGE HIGHER EFFLUENT SUSPENDED SOLIDS THAN FOUND UNDER CONVENTIONAL LOADINGS." |
| CONTACT STABILIZATION | Splits capture from metabolism into two tanks. "BEST APPLIED WHERE OTHER ACTIVATED SLUDGE MODES WOULD FAIL DUE TO THE SHORT SRTs AND DETENTION TIMES." Typically used in INDUSTRIAL APPLICATIONS OR SEVERELY OVERLOADED MUNICIPAL PLANTS that do not have enough aeration basin volume. |
The manual draws the distinction carefully: "THERE IS A RELATIONSHIP BETWEEN THE SRT AND THE F:M RATIO, ALTHOUGH THEY ARE NOT EXACTLY THE SAME THING."
SRT counts POUNDS OF MLSS against POUNDS OF SUSPENDED SOLIDS ENTERING.
F:M counts THE MASS OF LIVING MICROORGANISMS (MLVSS) against THE EDIBLE SOLIDS (BOD) that enter each day.
F:M is the more biologically meaningful of the two, which is why it uses VOLATILE suspended solids and BOD rather than total solids on both sides.
The manual notes the sequential batch reactor has BECOME VERY POPULAR RECENTLY. An SBR COMBINES THE AERATION BASIN AND THE SECONDARY CLARIFIER INTO A SINGLE BASIN. The basin fills over several hours while aerating; then aeration shuts off and the basin FUNCTIONS AS A SECONDARY CLARIFIER; then a DECANTING MECHANISM drains clarified water from the surface — which is when the system discharges effluent. Then the cycle repeats.
If you have a dog that weighs 100 lbs., it probably eats around 2 – 4 lbs. of dog food each day… we would say that the F:M of your dog ranges from 0.02 – 0.04 to 1.00.
— Chapter 10
| System | F:M | The analogy |
|---|---|---|
| A dog | 0.02–0.04 : 1 | A 100 lb dog eating 2–4 lbs a day |
| EXTENDED AERATION | 0.05–0.15 : 1 | 100 lbs of organisms eating 5–15 lbs per day |
| CONVENTIONAL | 0.25–0.5 : 1 | "Like a 100 lb. dog eating between 25 and 50 lbs. of food each day" |
| CONTACT STABILIZATION | 1.00 : 1 and beyond | "Like a 100 lb. dog eating 100 lbs. of dog food everyday!" |
"WHAT IS A STRANGE CONCEPT TO MANY PEOPLE WHEN CONSIDERING THIS ANALOGY IS THAT, IT IS NOT THE AMOUNT OF FOOD THAT AN ACTIVATED SLUDGE WASTEWATER OPERATOR IS IN CONTROL OF, IT IS THE SIZE OF THE DOG."
The food supply does not change much day to day. The operator cannot control what arrives at the plant.
What the operator controls is the MASS OF ORGANISMS — by increasing or decreasing the daily WAS flow. "OPERATORS EFFECTIVELY CONTROL THE SIZE OF THE DOG."
That reframing is the single clearest statement of what activated sludge process control actually is.
F:M = lbs/day BOD entering the aeration basin
÷ lbs MLVSS in the aeration basin
The manual’s worked example (extended aeration):
160 lbs/day BOD ÷ 2,000 lbs MLVSS = 0.08 F:M
MLVSS — VOLATILE suspended solids — is used because IT IS ASSUMED THAT ALL OF THE VOLATILE SOLIDS ARE COMPRISED OF LIVING MICROORGANISMS AND THE NON-VOLATILE SOLIDS ARE INERT MATTER THAT DOES NOT CONTRIBUTE TO METABOLIZING THE WASTE SOLIDS.
The manual gives a practical target: "TYPICALLY, THIS MEANS THAT THE NUMBER OF POUNDS OF SOLIDS WASTED FROM A SYSTEM EACH DAY MUST BE AROUND 50 – 70% OF THE TOTAL NUMBER OF POUNDS OF SOLIDS THAT ENTER THE SYSTEM EACH DAY."
The gap between 100% and that 50–70% is accounted for by SOLIDS DESTROYED THROUGH DIGESTION IN THE AERATION BASIN AND SOLIDS LOST TO THE EFFLUENT.
That is a useful sanity check on a wasting program even at a plant that cannot run daily MLVSS.
| What you see | F:M condition | What to do |
|---|---|---|
| CLEAR high quality effluent; SMALL AMOUNT OF CRISP WHITE FOAM; mixed liquor CHOCOLATE BROWN | F:M CLOSE TO IDEAL | Continue in the same manner. |
| Cloudy effluent; LARGE FLOC over the weirs (STRAGGLER FLOC); LOTS OF FROTHY WHITE OR GRAY FOAM; mixed liquor LIGHT BROWN OR TAN; BOD and TSS elevated | TOO HIGH — overloaded plant or start-up | REDUCE WASTING to build up a larger mass of MLSS. |
| THICK DARK FOAM; mixed liquor DARK BROWN OR DARK REDDISH; sludge FLOATING in the clarifier; VERY SMALL FLOC the size of a pin head (PIN FLOC) | TOO LOW | INCREASE WASTING. |
It lets an operator diagnose F:M condition from foam color, mixed liquor color and floc size — no laboratory required.
And the two error states have OPPOSITE corrections. STRAGGLER FLOC means waste LESS. PIN FLOC means waste MORE. Both produce a turbid effluent, and confusing them makes the problem worse in either direction.
The manual also gives the smell test: "THE SYSTEM SHOULD SMELL LIKE HEALTHY WET SOIL WHEN IT IS OPERATING WELL. FOUL ODORS INDICATE A LACK OF DISSOLVED OXYGEN, WHICH MEANS TROUBLE."
| Color | Meaning |
|---|---|
| LIGHT TAN OR YELLOW | The plant is probably IN START-UP. Does not yet have the healthy organic smell of ideal activated sludge. |
| CHOCOLATE BROWN | Ideal. |
| VERY DARK BROWN | OLD SLUDGE due to a high SRT. Common in small package plants that WASTE ONLY BY HAVING A SEPTIC HAULER REMOVE SLUDGE — solids build up for a long period between truck visits. |
| Accompanying sign | A SIGNIFICANT SCUM BLANKET usually accompanies dark brown mixed liquors — light brown to tan, or even DARK BROWN AND LEATHERY. |
| Problem | Detail |
|---|---|
| INSUFFICIENT MIXING | NO SETTLING SHOULD OCCUR IN THE BASIN. Evaluate with a stick or sludge blanket indicator by probing the bottom. Settled solids RAPIDLY BECOME SEPTIC, causing INCREASED OXYGEN DEMAND, LOWER DETENTION TIMES and EXCESS GROWTH OF FILAMENTOUS BACTERIA associated with septic conditions. |
| EXCESSIVE MIXING | If turbulence is too high, FLOC SHEAR occurs — floc particles are broken up. In the clarifier this leads to INCREASED EFFLUENT TSS. |
| Diagnosing floc shear | Under a MICROSCOPE the broken floc particles are evident. WITHOUT a microscope: LOOK FOR SIGNS OF EXCESSIVE TURBULENCE whenever effluent TSS seems unusually high without another obvious cause. |
The manual gives a specific and memorable case: "AN EXAMPLE OF THIS SITUATION IS WHEN RECREATIONAL VEHICLES (RVs) ARE ALLOWED TO DISCHARGE LARGE AMOUNTS OF HOLDING TANK WASTE TO A TREATMENT PLANT."
The mechanism: "CHEMICALS, SUCH AS FORMALDEHYDE, ARE OFTEN USED TO STABILIZE RV HOLDING TANKS. FORMALDEHYDE IS HIGHLY TOXIC TO ACTIVATED SLUDGE MICROBES, SO EVEN A SINGLE RV’S DISCHARGE CAN KILL-OFF A SMALL PACKAGE PLANT."
And the manual adds the constructive note: "MICROBE FRIENDLY, BIODEGRADABLE ALTERNATIVES ARE AVAILABLE AS A REPLACEMENT FOR FORMALDEHYDE BASED PRODUCTS."
For any plant with an RV dump station, that is a real operational and outreach issue.
In most municipal plants, influent flow and BOD/TSS DO NOT VARY BY MORE THAN 10% FROM DAY TO DAY — giving a relatively stable and predictable loading. The manual names the exceptions: a small package plant serving A SCHOOL, where flow occurs only 8:00 AM to 4:00 PM, stops entirely on weekends and for three months in summer; and a municipality with A FOOD PROCESSING PLANT causing a tremendous loading increase several days a week.
| Feature | Specification |
|---|---|
| Inlet flow control | Structures that let the operator CAREFULLY REGULATE THE HYDRAULIC LOADING to the clarifier. |
| Energy dissipating baffles | At the mixed liquor inlet, to QUICKLY SLOW THE MIXED LIQUOR AND DIRECT IT DOWNWARD. Gentle mixing during entry HELPS START BIO-FLOCCULATION. |
| SHORT-CIRCUITING | Should be eliminated. Causes include thermal density-currents and poor baffle design, "HOWEVER, THE MOST COMMON CAUSE IS UNEVEN WEIRS that draw supernatant over one area at a much higher rate." |
| DEPTH | Deep enough to allow some process upsets without loss of the sludge blanket — FOR MOST PLANTS, GREATER THAN 12 FEET. |
| DETENTION TIME | BETWEEN 2 AND 4 HOURS at the HIGHEST (PEAK) FLOW the clarifier will see. |
| SURFACE LOADING RATE | BETWEEN 300 AND 1,200 GALLONS PER DAY PER SQUARE FOOT. |
| Sludge removal | Effective removal for the ENTIRE bottom — typically a scraper sweeping toward the RAS pump inlet box. |
| RAS control | ACCURATE CONTROL OF THE PUMPING RATE, with some form of pump control and flow measurement. |
| Drains | Provided for each clarifier so they can be taken down for service and inspection. |
Simply increasing the RAS pumping rate for a sludge that will not settle will not bring the sludge blanket down (although this is the typical response by operators), because all of the sludge that is pumped out of the clarifier returns back to it.
— Chapter 10
The instinct when the blanket rises is to pump harder. The manual says plainly that this is THE TYPICAL RESPONSE BY OPERATORS and that it does not work for a sludge that will not settle — because RAS is a loop, not a removal.
Worse: "INCREASING THE RAS PUMPING RATE ABOVE THE ALLOWABLE RANGE OFTEN RESULTS IN CLARIFIER WASHOUT, BECAUSE AT SOME POINT, THE HYDRAULIC LOADING RATE OF THE CLARIFIER IS EXCEEDED."
So the wrong response to a settling problem can convert it into a solids loss event.
The manual’s conclusion: "THERE IS NO MAGIC SETTING FOR THE RAS PUMPING RATE, BUT RATHER A SERIES OF CHECKS AND OBSERVATIONS THAT OPERATORS MUST CONTINUALLY MAKE."
| Rule | Detail |
|---|---|
| The general aim | Run the RAS pumps JUST FAST ENOUGH TO MAINTAIN THE SMALLEST SLUDGE BLANKET POSSIBLE. |
| Blanket measurement | AT LEAST TWICE A DAY, measured at about the MIDDLE OF THE CLARIFIER BRIDGE. Core sampler or infrared detector — "THE METHOD USED IS LESS IMPORTANT THAN ENSURING THE MEASUREMENTS ARE PERFORMED IN A CONSISTENT MANNER." |
| The daily cycle | The typical 24-hour peak and low flow cycle GENERALLY CAUSES THE BLANKET TO ACCUMULATE THROUGHOUT THE DAY AND DROP THROUGHOUT THE NIGHT. |
| RISING SLUDGE | If sludge settles well but is not removed fast enough, biological activity continues and NITROGEN GAS BUBBLES form, floating particles and clumps to the surface. "WHENEVER THIS TYPE OF RISING SLUDGE IS OBSERVED, IT IS A SIGN THAT RAS PUMPING RATE SHOULD BE INCREASED." |
| Element | Detail |
|---|---|
| Purpose | A method of SIMULATING THE SETTLING OF ACTIVATED SLUDGE IN A SECONDARY CLARIFIER. |
| THE SAMPLE POINT | Mixed liquor taken FROM THE END OF THE AERATION BASIN, RIGHT BEFORE IT ENTERS THE SECONDARY CLARIFIER. |
| Sample size | Usually 1–2 liters. |
| The vessel | A large clear graduated beaker marked in mL/L and percent by volume. |
| THE SCHEDULE | Observe and record EVERY FIVE MINUTES FOR THE FIRST HALF HOUR, then at 60 MINUTES and at 120 MINUTES. |
| Graphing | Operators often graph the five-minute readings for the first 30 minutes, YIELDING A CHARACTERISTIC TYPE OF CURVE. |
The manual: "A SLUDGE THAT SETTLES TO AROUND 300 ml/L (OR 30%) WITH A CLEAR SUPERNATANT AT 30 MINUTES IS CONSIDERED IDEAL because it indicates that the sludge will settle rapidly and compact well in the secondary clarifier."
THE 30-MINUTE READING IS THE MOST USEFUL to operators for determining how well the sludge will settle.
And the honest corollary: "A SLUDGE THAT SETTLES SLOWLY AND DOES NOT COMPACT IN THE SETTLEOMETER, OR THAT LEAVES A CLOUDY SUPERNATANT, WILL PERFORM SIMILARLY IN THE SECONDARY CLARIFIER."
| Settleometer behavior | System condition |
|---|---|
| Settles and compacts RAPIDLY, leaves some PIN FLOC in the supernatant, often small amounts of FLOATING SLUDGE | LOW F:M and HIGH SRT |
| Settles and compacts MORE SLOWLY, leaves LARGE STRAGGLER TYPE FLOC and a slightly cloudy supernatant | HIGH F:M and LOW SRT |
| Settles to around 300 mL/L in 30 minutes with a CLEAR supernatant | CORRECT F:M AND SRT |
| SLUDGE RISES TO THE TOP within 120 minutes | THE SYSTEM IS ACTIVELY NITRIFYING — the sludge rises due to DENITRIFICATION. Ensure the RAS pumping rate is high enough to prevent rising sludge in the clarifier. |
The 30-minute reading tells you about settling. THE 120-MINUTE READING TELLS YOU WHETHER YOU ARE NITRIFYING.
A settleometer that rises within two hours is showing you denitrification happening in the beaker — the same thing that will lift sludge off your clarifier floor if RAS is too slow.
It is a free early warning, and it costs nothing but leaving the beaker on the bench.
| Item | Value |
|---|---|
| Secondary treatment removal | 90% or more of SS and BOD |
| Rock media size | 2–4 inches, UNIFORM |
| Rock media depth | 3–8 feet |
| Synthetic media depth | 15–30 feet |
| ROCK VOID SPACE | ~35% |
| PLASTIC VOID SPACE | ~95% |
| Standard-rate hydraulic | 25–100 gal/day/sq ft |
| Standard-rate organic | 5–25 lbs BOD/day/1,000 cu ft |
| Standard-rate effluent | BOD as low as 20–25 mg/L |
| High-rate hydraulic, rock | 100–1,000 gal/day/sq ft |
| High-rate hydraulic, synthetic | 350–2,100 gal/day/sq ft |
| High-rate organic, rock | 25–100 lbs BOD/day/1,000 cu ft |
| High-rate organic, synthetic | 50–300 lbs BOD/day/1,000 cu ft |
| Roughing filter | 100 to over 300 lbs BOD/day/1,000 cu ft |
| Roughing filter removal | 50–70% BOD |
| Ponding — chlorine dose | About 5 mg/L for several hours |
| Ponding — flooding | Submerged 24 hours |
| FILTER FLY LIFE CYCLE | As short as SEVEN DAYS |
| Distributor speed | ~1 RPM on larger units |
| Distributor flushing | Weekly, end dump gates ONE AT A TIME |
| Poor effluent diagnostic | High TOTAL BOD = escaping solids; high SOLUBLE BOD = capacity exceeded |
| Item | Value |
|---|---|
| Removal capability | BOD and TSS in excess of 99% not unusual |
| Minimum D.O. | 1.0 mg/L — but F:M dependent |
| Conventional — detention | 4–8 hrs |
| Conventional — MLSS | 1,000–4,000 mg/L |
| Conventional — SRT | 3.5–10.0 days |
| Conventional — F:M | 0.25–0.5 : 1 |
| Conventional — RAS | 15–75% |
| Extended aeration — detention | 12–24 hrs |
| Extended aeration — MLSS | 2,000–5,000 mg/L |
| Extended aeration — SRT | >10 days |
| Extended aeration — F:M | 0.05–0.15 : 1 |
| Extended aeration — RAS | 50–150% |
| Contact stabilization — SRT | <3.5 days |
| Contact stabilization — F:M | 0.5 to over 1.0 : 1 |
| Contact stabilization — RAS | 25–100% |
| General RAS range | 25–150% of influent flow |
| DAILY WASTING TARGET | 50–70% of lbs solids entering per day |
| F:M formula | lbs/day BOD ÷ lbs MLVSS |
| SRT formula | lbs MLSS ÷ lbs SS entering per day |
| Clarifier depth | Greater than 12 feet |
| Clarifier detention at peak | 2–4 hours |
| CLARIFIER SURFACE LOADING | 300–1,200 gal/day/sq ft |
| Blanket measurement | At least TWICE A DAY, middle of the bridge |
| SETTLEOMETER TARGET | ~300 mL/L at 30 minutes, CLEAR supernatant |
| Settleometer schedule | Every 5 min for 30 min, then 60 and 120 min |
| Rise within 120 minutes | System is NITRIFYING |
| Straggler floc | F:M TOO HIGH — waste LESS |
| Pin floc | F:M TOO LOW — waste MORE |
| Ideal mixed liquor | CHOCOLATE BROWN, crisp white foam |
| Healthy smell | Like HEALTHY WET SOIL |
New Mexico Wastewater Systems Operator Certification Study Manual, CHAPTER 7: FIXED FILM SECONDARY TREATMENT and CHAPTER 10: ACTIVATED SLUDGE. Prepared by the Water Utilities Technical Assistance Program (NMSU DABCC WUTAP) for the Facility Operations Section, NMED Surface Water Quality Bureau, January 2005.
CHAPTER 7 content used: the definition of secondary treatment and the 90% removal figure; trickling filter description, the three basic parts, principles of the treatment process and principles of operation; media types, sizes, depths and void space percentages; ventilation requirements; recirculation purposes and the weekly high-rate sloughing practice; filter classification (standard-rate, high-rate, roughing) with hydraulic and organic loading ranges; filter staging; operational strategy; and the response-to-abnormal-conditions material covering ponding, odors, filter flies, sloughing, poor effluent quality, cold weather problems and maintenance of bearings, seals and distributor arms.
CHAPTER 10 content used: process description; the four operator controls; step feeding; dissolved oxygen and mixing; RAS pumping rate control; mixed liquor concentration and the F:M relationship; the three modes of operation with full operating parameter sets; SBRs; process control covering influent characteristics, toxic substances, the aeration basin environment, mixing and floc shear, the F:M dog analogy and calculation, the visual diagnosis table, secondary clarifier design features, RAS flow control, the settleometer test, and the mixed liquor color and odor diagnostics.
SECTIONS NOT COVERED HERE: Chapter 7’s ROTATING BIOLOGICAL CONTACTORS section (media observation, black and white appearance, sloughing, maintenance); Chapter 10’s MECHANICAL COMPONENTS (aeration systems, blowers, air headers, diffusers, filters, and the associated safety sections); and Chapter 10’s later PROCESS CONTROL STRATEGIES material on MLSS/MLVSS, SVI, MCRT, light microscopy, solids separation problems, RAS chlorination, foaming, and responding to plant changes. SAY THE WORD AND I WILL BUILD A SECOND GUIDE ON THOSE.
FIGURES AND TABLES NOT EXTRACTED: several figures in both chapters are images — including Figure 10.14 (Settleability Curve) and Figure 10.5 (Settleable Solids). Their contents are not reproduced. Available on request via page rasterization.
The YOUNG VS OLD SLUDGE guide — the straggler floc versus pin floc distinction, the mixed liquor color spectrum, and the foam characteristics all appear here in the manual’s own words, corroborating that framework directly.
The SOLIDS HANDLING guide — WAS produced here is the feed to Chapter 11, and the manual’s note that extended aeration sludge arrives ALREADY PARTIALLY STABILIZED explains why it digests differently.
The ALKALINITY guide — the settleometer rise within 120 minutes as a nitrification indicator connects to the nitrification and alkalinity relationship, since a nitrifying system is consuming 7.14 mg/L alkalinity per mg/L ammonia oxidized.
The INFLUENT VS EFFLUENT guide — the manual’s emphasis on accurate influent flow measurement and periodic influent BOD and TSS sampling is the input side of the percent removal framework.
The TERTIARY TREATMENT guide — Chapter 12 follows directly, and LUXURY UPTAKE phosphorus removal depends entirely on a well-controlled activated sludge process of the kind described here.
| Item | Note |
|---|---|
| DATE | January 2005. Process fundamentals and operating parameters are durable; verify any regulatory content separately. |
| Scope | Chapters 7 and 10 partially. RBCs, mechanical components, and the SVI/MCRT/microscopy/troubleshooting material are not covered here. |
| Figures | Several are images and were not extracted. |
| Operating ranges | Typical values as the manual states them. Your plant’s design basis governs. |
| "Most common in New Mexico" | Some statements are state-specific. |
| Not a design document | Operator guidance from a certification study manual. |
| SAFETY | Chapter 10 contains substantial safety material on surface aerators, blowers and air distribution that is NOT reproduced here. Read it in the original. |
The manual’s own hierarchy is clear and worth adopting: "IT IS THE F:M RATIO THAT MUST FIRST BE DETERMINED in order to understand what mode of operation the system is in. ONLY AFTER THE F:M IS UNDERSTOOD CAN THE OTHER OPERATING FACTORS BE ASSESSED."
The most useful thing for a working operator is the visual diagnosis table in Part 7 — foam, mixed liquor color and floc size give you the F:M condition with no lab data, and the two error states have OPPOSITE corrections.
And the correction worth putting in front of every operator: pumping RAS harder does not fix a sludge that will not settle. It is a loop, not a removal — and pushed far enough it causes clarifier washout.
The full treatment process, step by step · The activated sludge guide · SVI and the SVI calculator · Tertiary treatment · The Grade 2 exam review
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