Every diagram on this site was drawn for ActivatedSludge.com. Teachers and operators may use them for in-plant training with credit to ActivatedSludge.com.
Aeration basin cross section Primary effluent and return activated sludge enter an aeration basin. A blower feeds fine bubble diffusers on the floor. Mixed liquor flows out to the secondary clarifier. Blower Primary effluent RAS Mixed liquor to clarifier Aeration basin (aerobic zone) Fine bubble diffuser grid on the floor Air supplies oxygen and keeps the floc in suspension Floc + bubbles DO 1.5 to 2.5 mg/L Aeration basin. Read the related page Secondary clarifier zones Mixed liquor enters a center feed well, settles through a clear zone and settling zone, and thickens in a sludge blanket on the floor. Clear effluent flows over peripheral weirs; settled sludge is withdrawn as RAS and WAS. Mixed liquor in (center feed) Effluent weir and launder RAS / WAS Scum Clear water zone Settling zone Sludge blanket (thickening zone) Secondary clarifier zones. Read the related page RAS and WAS relationship Influent enters the aeration basin, mixed liquor flows to the clarifier, effluent leaves. Settled sludge returns to the aeration basin as RAS, and a smaller stream is removed as WAS to control sludge age. Influent Aeration basin MLSS 2,500 to 3,500 mg/L Secondary clarifier Effluent RAS: returns settled organisms (often 40 to 100% of Q) WAS to thickening RAS controls where the solids are. WAS controls how many there are (SRT). RAS and WAS relationship. Read the related page Solids retention time concept The solids inventory in the aeration basin is divided by the pounds per day leaving through waste sludge and effluent. The result is the average number of days a particle of sludge stays in the system. Solids inventory lb under aeration Growth from food WAS, lb/d (operator controlled) Effluent TSS, lb/d (loss) SRT, days = inventory, lb / (WAS lb/d + effluent lb/d) Example: 90,072 lb / 7,740 lb/d = 11.6 days Solids retention time. Read the related page Food to microorganism ratio A balance compares the pounds of BOD fed each day with the pounds of volatile solids under aeration. Typical F/M ranges for extended aeration, conventional and high rate systems are shown. Food (F) BOD, lb/d Microorganisms (M) MLVSS, lb F/M = lb BOD per day / lb MLVSS under aeration Extended aeration 0.05 to 0.15 Conventional 0.2 to 0.4 High rate 0.4 to 1.5 Nitrifying plants usually run lower F/M (longer SRT) than BOD-only plants. Food to microorganism ratio. Read the related page Nitrification pathway Ammonium is oxidized to nitrite by ammonia oxidizing bacteria and then to nitrate by nitrite oxidizing bacteria. The process needs oxygen, alkalinity, adequate SRT and temperature. NH₄⁺ ammonium NO₂⁻ nitrite NO₃⁻ nitrate AOB (Nitrosomonas) NOB (Nitrobacter, Nitrospira) Needs: oxygen (4.57 lb O₂ per lb N), SRT, warm water, pH 7 to 8 and alkalinity (7.14 mg/L as CaCO₃ consumed per mg/L N) Slow growers: doubling time of a day or more in cold water Washout happens if SRT drops below what the temperature requires Nitrification pathway. Read the related page Denitrification and the MLE process Nitrate is reduced stepwise to nitrogen gas. In a Modified Ludzack-Ettinger layout, an internal recycle returns nitrate from the aerobic zone to an anoxic zone at the front where influent carbon drives denitrification. NO₃⁻ NO₂⁻ NO N₂O N₂ gas Heterotrophs use nitrate instead of oxygen when DO is near zero and carbon (BOD) is available Returns 3.57 mg/L alkalinity per mg/L nitrate-N and saves 2.86 lb O₂ per lb N Anoxic zone DO < 0.2 mg/L, mixed Aerobic zone nitrification Clarifier Internal recycle (nitrate), 200 to 400% of Q RAS Denitrification and MLE. Read the related page Three-stage BNR layout Anaerobic, anoxic and aerobic zones in series with a nitrate recycle and RAS. Phosphorus accumulating organisms release phosphorus in the anaerobic zone and take up more in the aerobic zone. Anaerobic no DO, no nitrate Anoxic nitrate, no DO Aerobic DO 1.5 to 2.5 Clarifier Nitrate recycle RAS P release, VFA uptake Denitrification Nitrification, P uptake A2O style layout: biological nitrogen and phosphorus removal Three-stage BNR layout. Read the related page Floc formation Dispersed bacteria clump together with extracellular polymer into small flocs, then into mature flocs supported by a few short filaments. Dispersed bacteria young sludge, cloudy Small floc forming EPS glues cells together Mature floc firm, settles well, short filaments as backbone Floc formation: from single cells to settleable floc Floc formation. Read the related page Filamentous bulking compared with good settling Two settleometers: one settles to 280 mL/L, one with filamentous bulking only to 820 mL/L. Microscope sketches show compact floc compared with filaments bridging between flocs. 280 Good settling SSV30 280, SVI about 95 820 Filamentous bulking SSV30 820, SVI about 280 Compact floc Filaments bridge flocs Bridging holds flocs apart, so the sludge cannot compact in the clarifier. Filamentous bulking. Read the related page Oxygen transfer basics Oxygen moves from an air bubble through gas and liquid films into the mixed liquor. The driving force is the gap between saturation and the operating DO. Fine bubbles transfer more oxygen than coarse bubbles. Air bubble 21% oxygen Gas film and liquid film at the surface Driving force C* (saturation) DO in tank Transfer ∝ alpha x (beta C* - DO) Fine bubble more surface, SOTE 20 to 35% Coarse bubble less surface, SOTE 8 to 15% Lower DO in the tank means more driving force but less margin for the bugs. Oxygen transfer. Read the related page State point analysis diagram A gravity solids flux curve with an overflow line from the origin and an underflow line through the state point. Here the underflow line stays below the descending limb, so the clarifier is underloaded. 0 2 4 6 8 10 12 14 0 1 2 3 4 5 6 State point Overflow line, slope = Q/A Underflow line, slope = -RAS/A Gravity flux curve RAS conc Solids concentration, g/L Solids flux, kg/m²·h State point analysis. Read the related page Sludge blanket depth Three clarifiers show a normal blanket of 1 to 3 feet, a rising blanket of 4 to 6 feet that needs action, and a blanket near the weirs about to lose solids. A core sampler measures depth. Normal blanket 1 to 3 ft Rising blanket 4 to 6 ft: act Near loss solids at the weirs Core sampler or blanket finder Sludge blanket depth. Read the related page Preliminary treatment train Influent passes a bar screen, a grit removal unit and a flow measuring flume. Influent Bar screen rags, plastics Grit removal sand, gravel Flow measurement Parshall flume Preliminary treatment protects everything downstream Screenings to landfill Grit washed, to landfill Preliminary treatment. Read the related page Primary clarifier A primary clarifier settles raw solids and floats grease. Primary sludge is pumped from the floor. Scum and grease skimmed About 2 hours detention removes 50 to 65% of TSS and 25 to 35% of BOD Primary sludge Primary clarifier. Read the related page Disinfection Clarified effluent passes UV lamps or a chlorine contact tank before discharge. Clarifier effluent UV channel (or chlorine contact tank) Effluent to receiving water Disinfection. Read the related page Solids handling train Waste and primary sludge are thickened, digested, dewatered and sent out as biosolids. WAS and primary sludge Thickening GBT or gravity Anaerobic digestion Dewatering centrifuge Biosolids land application Solids handling: thicken, stabilize, dewater Recycle streams (filtrate, centrate) return ammonia and solids to the headworks Solids handling train. Read the related page Protozoa groups Six panels: an amoeba with pseudopods, small flagellates with whip like flagella, an oval free-swimming ciliate covered in cilia, a crawling ciliate walking on floc, two bell shaped stalked ciliates on a floc, and a suctorian with knobbed tentacles on a stalk. The six protozoa groups operators track Amoebae Flowing pseudopods, slow Young sludge, startup Flagellates 5 to 20 microns, whip like tail Young sludge, high soluble BOD Free-swimming ciliates Covered in cilia, fast Younger sludge, dispersed bacteria Crawling ciliates Walk on floc with cirri Stable, well flocculated sludge Stalked ciliates Bells on stalks, ring of cilia Mature, stable sludge Suctorians Tentacles catch other protozoa Mature, older sludge Green text: what a large number of each group usually suggests. Line drawings, not to scale. Protozoa groups. Read the related page Higher life forms Four panels: a rotifer with a two lobed ciliated crown and forked foot, a smooth nematode worm, a segmented Aeolosoma bristle worm with bristle tufts and colored spots, and a water bear with eight clawed legs. Higher life forms in activated sludge Rotifer 100 to 500 microns Ciliated crown, forked foot Nematode 0.5 to 3 mm Smooth roundworm, thrashes Bristle worm Aeolosoma, up to 10 mm Segments, bristle tufts, color spots Water bear Tardigrade, 0.1 to 1 mm Eight stubby clawed legs All are multicellular and usually point to an older, stable sludge with a longer SRT. Higher life forms. Read the related page Naked and testate amoebae Left: a naked amoeba with pseudopods, nucleus, food vacuoles and a contractile vacuole. Right: two testate amoebae, a round brown Arcella shell and a vase shaped Euglypha shell made of plates, with pseudopods extending. Naked and testate (shelled) amoebae pseudopod nucleus food vacuoles contractile vacuole Arcella (side view) Euglypha Naked amoeba: young sludge, startup, recovery Slow, shape changing, easy to miss at 100x Testate amoebae: long SRT, low F/M Often with full nitrification Naked and testate amoebae. Read the related page Flagellate size compared with floc A microscope field with a floc and five tiny flagellates around it, with one flagellate enlarged to show its oval body and whip like flagellum. Flagellates are tiny: look at 400x with phase contrast cell body 5 to 20 microns flagellum Darts or spins; some are green (Euglena) about 100 microns Floc in the view: 100 to 500 microns A flagellate is a speck next to it at 100x Flagellate size compared with floc. Read the related page Inside a healthy floc An irregular floc made of floc forming rod bacteria held together by sticky polymer, with a few short filaments running through it as a backbone, some trapped particles and stalked ciliates on its edge. The liquid around it is clear. Inside a healthy floc Filament backbone gives strength Floc forming bacteria (mostly rods) EPS: sticky polymer glue Trapped particles and fines Stalked ciliates graze on the edge Clear liquid, few dispersed cells Size: about 50 to 1,000 microns Firm, irregular, a few internal filaments, clear background: settles well and leaves a clear supernatant. Inside a healthy floc. Read the related page Healthy floc, filamentous bulking and pin floc compared Three microscope views with settleometers: healthy floc with a few internal filaments settles well with a clear supernatant; filamentous bulking with filaments bridging flocs settles slowly to a high blanket; pin floc of tiny particles settles fast but leaves a turbid supernatant. Three floc conditions compared Healthy floc Filamentous bulking Pin floc Settles well, clear supernatant SVI usually 80 to 150 Filaments mostly inside floc Settles slowly, compacts poorly High SVI, blanket may rise Filaments bridge between flocs Settles fast, turbid supernatant Low SVI, cloudy effluent Few or no filaments, long SRT Drawings show a 100x view and a 30 minute settleometer for each case. Not to scale. Healthy floc, bulking and pin floc. Read the related page Dispersed growth compared with healthy floc Left: a firm floc in clear liquid with a stalked ciliate, and a settleometer with a clear supernatant. Right: many single bacteria and tiny clumps with a free-swimming ciliate and a flagellate, and a settleometer that stays cloudy. Dispersed growth compared with healthy floc 30 min 30 min Healthy floc Firm floc, clear liquid, stalked ciliates Clear supernatant Dispersed growth Single cells and tiny clumps, free swimmers and flagellates Cloudy supernatant that does not clear Dispersed growth compared with healthy floc. Read the related page Capsules and sheaths Left: rod shaped bacteria each surrounded by a clear capsule of slime. Right: a sheathed filament with cells inside a clear tube, a false branch and an empty section of sheath. How bacteria protect themselves: capsules and sheaths capsule cell empty sheath false branch cells inside a sheath Capsule (slime layer) Shows as a clear halo in India ink Sheath A clear tube around a filament, as in S. natans Capsules and sheaths. Read the related page Bacterial shapes Three microscope fields: round cocci as singles, pairs, chains, clusters and tetrads; rod shaped bacilli single and in chains; and spiral shaped bacteria including a comma shaped vibrio. Bacterial shapes seen at 1000x tetrad single, pair chain cluster rods in a chain vibrio Coccus (round) Singles, pairs, chains, clusters, tetrads Bacillus (rod) Most floc formers are rods Spiral Spirillum, and the comma shaped vibrio Most bacteria are 0.5 to 3 microns, too small to see well below 400x. Bacterial shapes. Read the related page Aerobic respiration Organic food and dissolved oxygen enter a bacterium. It produces carbon dioxide, water, new cells and energy. Aerobic respiration: what a floc forming bacterium does with food and oxygen Bacterium oxidizes the food Organics BOD (food) O2 Oxygen from aeration CO2 carbon dioxide H2 O water New cells sludge growth (WAS) Energy powers the cell Organics + O2 → CO2 + H2 O + new cells + energy Aerobic respiration. Read the related page
