Fixed-film, lagoon and SBR treatment

# Fixed-film, lagoon and SBR treatment

> Trickling filters, RBCs, lagoons and SBRs all treat wastewater without the usual aeration-tank-and-clarifier pair. The exam wants you to know how each one works and where it goes wrong.

Why the exam tests this

Treatment Process (38%) is the exam domain fixed-film, lagoon and SBR content falls under. Most small operators run one process day to day. A lagoon operator may never touch an RBC, and a trickling-filter operator may never run an SBR.

The exam does not know which plant you work at, so it tests all three. A Class I-IV license lets you run any of them.

The processes also get confused with each other and with activated sludge. Fixed-film media, lagoon oxygen types and SBR cycle steps look similar on paper but run on different depths, loadings and timing.

What you need to know

Trickling filters and RBCs (the fixed-film basics)

Fixed-film processes grow bacteria on media instead of keeping them suspended in a tank. That gets you lower energy use and better recovery from shock loads than activated sludge. The tradeoff is more land, weaker cold-weather performance, and occasional odor.

A trickling filter sprays wastewater over rock or plastic media and lets it trickle to an underdrain below. A slime layer of bacteria grows on the media and eats the organic matter as it passes. That layer periodically sloughs off in chunks, and the sloughed biomass settles out in a secondary clarifier.

Rock beds use 2 to 3 inch media and run 6 to 10 feet deep. Plastic tower filters, called biotowers, go up to 30 feet. Filters stay reliable above about 55°F; sloughing tends to happen at seasonal temperature swings.

An RBC does the same job in reverse: it moves the media through the wastewater instead of moving the water over fixed media. Plastic discs, usually 12 feet across, sit on a shaft and rotate at about 1.5 rpm. Roughly 40 percent of the media sits underwater at any moment, so each rotation dips the slime for food, then lifts it for oxygen.

A shaft runs to a maximum of 27 feet, with media on at most 25 feet of it. RBC trains typically split into four baffled stages, and each stage acts as its own complete-mix chamber. BOD removal runs heaviest in the first stage, and nitrification starts as BOD drops through the later ones.

Denser media, usually high-density polyethylene, packs more surface area onto the same shaft length. Plants use it in the later, nitrifying stages, where surface area does the most work.

At higher grades, RBC organic loading has a working ceiling worth memorizing. Design loading typically runs 2.5 to 4 pounds of BOD per day per 1,000 square feet of media, with 6.4 as the hard limit. Above about 6 pounds, the sulfur filament Beggiatoa takes over, showing up as a white or gray biomass under low dissolved oxygen.

Media color reads the stage at a glance:

• Gray, shaggy slime: BOD removal underway.
• Brown, thinner slime: nitrification underway.
• White: high organic loading.
• Black: high loading, low DO, odor.

Lagoons

About a quarter of community wastewater systems, mostly small ones, run a lagoon. A lagoon does primary and secondary treatment in one slow pond, holding wastewater for weeks. Solids come out only every 10 to 20 years.

The tradeoff is land. Lagoons need a lot of it, and cold weather slows the biology down.

Lagoons split into three types by oxygen. Aerobic ponds carry dissolved oxygen through most of the depth. Anaerobic ponds carry none and usually sit first in a series, while facultative ponds run an aerobic top layer over an anaerobic bottom.

A facultative pond's layers flip in spring and fall, when the water turns over. That stirs up the bottom and brings a stretch of turbidity and odor until the pond restratifies.

Depth varies by type. Un-aerated aerobic lagoons run 3 to 8 feet deep. Aerated lagoons run 10 to 15 feet, and complete-mix aerated lagoons, which hold oxygen through the whole depth, run 1 to 6 feet.

On a partial-mix lagoon, run the aerators at night. Algae make oxygen during daylight, and leaving the aerators on then wastes power blowing that oxygen back out.

Most lagoon systems run several cells in series, in parallel, or both. The last cell is usually lightly loaded for polishing, sometimes a constructed wetland.

Field thresholds tell you when to act:

• Desludge when average sludge depth reaches 25 percent of operating depth (a 60-inch lagoon desludges at 15 inches).
• Target DO in the upper sunlit layer runs about 2.0 mg/L; step in at 1 mg/L or below, or if pH drops below 6.5.
• Weed cover past 40 percent of the surface calls for removal.

Sequencing batch reactors

An SBR runs activated sludge in time instead of space, cycling one tank through five steps. Fill, react, settle, decant, and idle happen in order, then repeat.

Fill can run three ways:

• Static: no mixing or air, which saves power.
• Mixed: anoxic drives denitrification; anaerobic releases phosphorus for later uptake.
• Aerated: air runs during filling.

React handles most of the carbon removal. Settle has to stay quiescent, since poor settling ruins the decant that follows. Decant pulls off the clear water on top, capped at one-third of basin volume, and idle is when you waste sludge.

A well-run decanter lets an SBR skip a separate clarifier and still hit TSS under 10 mg/L. Cycling the tank between aerobic, anoxic and anaerobic conditions also gets nutrient removal without extra tanks: BOD under 5, total nitrogen under 5, and phosphorus under 2 mg/L are achievable in the right design.

At higher grades, design and control add detail. Run at least two SBR basins for redundancy, plus an influent equalization basin, and favor flow-paced operation over time-paced so every cycle handles the same load. During aerated fill, hold DO at or below 0.2 mg/L to protect the anoxic conditions idle depends on, and keep alkalinity 40 to 70 mg/L as CaCO3 before decant, with at least 50 mg/L left after.

Worked examples

RBC organic loading

An RBC's first stage carries 150,000 square feet of media. The stage gets 0.4 MGD of flow at 180 mg/L BOD. Find the loading in pounds of BOD per day per 1,000 square feet.

Convert the flow and concentration to a daily load: 180 mg/L x 0.4 MGD x 8.34 = 600.5 pounds of BOD per day. Divide by the media area in thousands of square feet, 150,000 divided by 1,000 equals 150. 600.5 divided by 150 equals 4.0 lb BOD per day per 1,000 square feet. That sits right at the top of the typical 2.5 to 4 range. It is worth watching, but not yet into Beggiatoa territory. Check your own loading math at /tools/practice.

RBC hydraulic loading

A four-stage RBC train carries 250,000 square feet of total media and treats 0.4 MGD, with a permit that requires ammonia removal along with BOD. Find the hydraulic loading in gallons per day per square foot.

0.4 MGD is 400,000 gallons per day. 400,000 divided by 250,000 equals 1.6 gpd/ft2. That falls inside the 1.5 to 1.8 gpd/ft2 band required for BOD plus ammonia removal, with little room to spare if flow climbs. Run the same division on your own numbers at /tools/practice.

Lagoon desludge check

A lagoon cell runs at a 6-foot, or 72-inch, operating depth. A sludge survey averaged across the grid points finds 20 inches of accumulated sludge. Does the cell need desludging?

20 divided by 72 equals 0.278, or about 28 percent of operating depth. That is past the 25 percent trigger, so the cell is due for desludging. Practice this kind of percent-of-depth check at /tools/practice.

Common traps

• Reading RBC organic loading in pounds per acre instead of pounds per 1,000 square feet. The units drive the whole loading check.
• Assuming aerated and un-aerated lagoons run the same depth. Un-aerated aerobic ponds stay shallow, 3 to 8 feet; aerated ponds run deeper, 10 to 15 feet, to hold the mixing energy.
• Running a partial-mix lagoon's aerators during the day. That off-gasses the oxygen algae are making and wastes power.
• Treating SBR aerated-fill DO the same as react-phase DO. Fill needs DO held low, at or below 0.2 mg/L, to protect the anoxic step ahead.
• Assuming every plant runs primary treatment before secondary. A lot of SBR and lagoon systems let the biological stage handle everything and never build a primary step at all.
• Reading an RBC's brown, thinner slime as a failing process. It signals nitrification underway, not trouble.

Practice

Fixed-film loading, lagoon field checks, and SBR cycle steps make up this duty area's share of the wwt-1 set, worked at /tools/practice. Drill the RBC organic and hydraulic loading math until the unit conversions are automatic. Then work through the lagoon desludge and DO thresholds, and retake missed questions a few days later to confirm the numbers stuck.

Quick reference

• Fixed-film grows bacteria on media (trickling filters, RBCs); lower energy use, better shock-load recovery, more land needed, weaker in cold weather.
• Trickling filters: rock media 2 to 3 inches, 6 to 10 feet deep; biotowers (plastic media) up to 30 feet; reliable above about 55°F.
• RBCs: discs about 12 feet across, about 1.5 rpm, about 40 percent submerged; shaft maximum 27 feet, media on at most 25 feet.
• RBC organic loading: typical 2.5 to 4, ceiling 6.4, pounds of BOD per day per 1,000 square feet; above about 6, Beggiatoa takes over.
• RBC hydraulic loading: 1.5 to 6.0 gpd/ft2 for BOD only; 1.5 to 1.8 gpd/ft2 when ammonia removal is also required.
• Lagoon types by oxygen: aerobic (dissolved oxygen through the depth), anaerobic (none, usually first cell), facultative (aerobic top, anaerobic bottom).
• Lagoon depths: un-aerated aerobic 3 to 8 feet; aerated 10 to 15 feet; complete-mix aerated 1 to 6 feet.
• Lagoon desludge trigger: average sludge depth at or above 25 percent of operating depth.
• Lagoon DO target: about 2.0 mg/L in the upper layer; step in at 1 mg/L or below, or pH below 6.5.
• SBR cycle, in order: fill, react, settle, decant, idle. Decant caps at one-third of basin volume.
• SBR aerated-fill DO: hold at or below 0.2 mg/L to protect the anoxic step.
• SBR alkalinity: 40 to 70 mg/L as CaCO3 before decant, at least 50 mg/L residual after.