Preliminary and primary treatment

# Preliminary and primary treatment

> Before the bugs in the aeration tank ever see the water, the headworks and the primary tanks pull out what would wreck everything downstream. Both stages get tested here in the same order they run in the plant.

Why the exam tests this

You already know what your own headworks and primary tanks look like on a normal day, screen and clarifier included. The exam does not ask about your plant. It asks for the removal ranges, the stage order, and why each piece of equipment sits where it does.

Untreated wastewater carries pathogens and organic material that starves a receiving stream of oxygen. The headworks and primary tanks are the first physical barrier against that. That is part of why the exam holds you to more than pattern recognition here.

That gap trips people up. Headworks and primary treatment feel like the boring part of the plant, before the biology starts. The exam treats them as part of the Treatment Process duty, worth 38 percent of the Class I exam. It expects you to reason about the stages, not just recognize them.

What you need to know

A mechanical plant's train runs preliminary, then primary, then secondary treatment. Preliminary treatment, the headworks, protects everything downstream by removing the coarse and the abrasive before it can damage a pump or clog a tank.

Screening comes first. Wastewater passes through parallel bars that hold back rags, sticks, cans, and plastic. Small plants rake screens by hand; larger ones rake mechanically, and screenings go to landfill, removed from the stream entirely.

Some plants grind instead of screening, using comminutors, macerators, or hammermills. Grinding does not remove anything from the stream, it shreds material small enough to pass through. A grit chamber usually sits ahead of the grinder to protect it.

Grit removal is next: sand, gravel, cinders, eggshells, and coffee grounds that would abrade pumps and fill up tanks. A grit channel is sized to hold velocity at about 1 foot per second.

That speed lets heavy inorganic grit settle out while lighter organic material stays suspended and moves on. At Class I, you need to recognize that fact.

At Class II and up, you apply it. A channel running faster than that lets grit escape; one running slower starts settling organics you want to keep moving.

A separate design reference gives grit-channel velocity as 15 to 30 centimeters per second, in the same general range.

Screenings and grit both go to landfill, not to the digester with primary sludge. They are inert or non-biodegradable, so mixing them into the biological or digestion side just wastes capacity.

The headworks may also handle flow equalization, softening the peaks before they hit the biology. In-line equalization passes the whole flow through a holding basin; off-line equalization diverts only the flow above a set point. Either way, the plant never has to size its downstream process for the full peak.

That matters twice over: it protects the downstream biology from shock loads, and it lets the plant get away with building a smaller secondary process.

Some headworks also handle septage receiving and odor control.

Primary treatment comes next. Wastewater moves slowly through primary clarifiers, sedimentation tanks sized to give heavy settleable solids time to sink.

The fraction a primary clarifier catches is the settleable solids, measured by settling a sample in an Imhoff cone. Colloidal and dissolved material do not settle out here. They move on to secondary treatment, along with anything still suspended when the water leaves the tank.

What settles forms a sludge blanket on the bottom. What floats, grease and scum, gets pulled off by a skimmer. Most plants run at least two primary tanks, so one can come offline for cleaning or repair without stopping the plant.

Some plants add a coagulant to help fine particles settle faster, though most primary clarifiers work by gravity alone.

A primary clarifier working normally removes 50 to 70 percent of suspended solids and 20 to 40 percent of BOD. Settled primary sludge gets pumped to digestion, and it carries a real load. As much as a third of the incoming BOD can end up in that primary sludge instead of reaching the biology.

If primary sludge runs thin and watery instead of thick, the draw-off rate is too high. It is pulling more water than solids.

At Class II and up, read that signal and act on it. Don't just recall the definition of a clarifier.

Not every plant has this step. Many small sequencing batch reactor and lagoon plants skip primary settling entirely and let the biological stage handle everything.

Worked examples

Example 1: Suspended solids removal. A clarifier's influent runs 220 mg/L TSS, a medium-strength value. Suppose the clarifier removes 60 percent, inside the normal 50 to 70 percent band. Effluent TSS works out to 220 x (1 - 0.60) = 88 mg/L.

That 88 mg/L answer comes from one relation, worked either direction: what came out subtracted from what went in, that difference divided by what went in, then times 100. That relation gets its own full workout, with more examples, on the wastewater treatment math guide; check your arithmetic on the practice calculator.

Example 2: BOD removal. The same influent runs 220 mg/L BOD, also medium strength. At 32 percent removal, inside the 20 to 40 percent band for BOD, effluent BOD works out to 220 x (1 - 0.32) = about 150 mg/L. If a result on your exam lands outside either band, recheck the math before you second-guess the plant.

Example 3: Reading the sludge draw. An operator pulls primary sludge for the digester and it runs thin. It looks closer to raw wastewater than to a proper sludge.

The fix is not to run the pump longer. Cut the draw-off rate back and let more solids concentrate before the next pull.

Common traps

• Confusing grit removal with primary clarification. Grit removal takes out sand and heavy inorganic material; it is not where BOD or organic solids come out.
• Assuming grinding removes material from the stream. Comminutors and macerators shred solids so they pass through; they do not take anything out.
• Picking a single number instead of the range. Primary removal runs 50 to 70 percent TSS and 20 to 40 percent BOD, not one fixed figure.
• Misreading a thin, watery sludge draw as good performance, when it signals the draw-off rate is too high.
• Assuming every plant has a primary clarifier. Small SBR and lagoon plants commonly skip that stage.
• Assuming primary treatment removes everything suspended. It only catches the settleable fraction; colloidal and dissolved material pass on to secondary treatment.

Practice

Work the preliminary and primary treatment questions in the wwt-1 practice test at the practice tool. Drill the percent-removal math in both directions, solving for effluent from a removal percentage and for removal percentage from influent and effluent.

Drill the headworks equipment order until you can name what comes before what without stopping to think. If a removal result lands outside the stated bands, recheck your math before you change anything on the plant side. Pay attention to which fraction of solids each stage removes, since the exam likes to test that distinction directly.

Quick reference

• Headworks order: screening or grinding, then grit removal, then (where present) flow equalization, septage receiving, and odor control.
• Screening removes rags, sticks, cans, and plastic to landfill; grinding shreds solids instead of removing them.
• A grit chamber usually sits ahead of a grinder to protect it.
• Grit channel design velocity: about 1 foot per second, so grit settles while organics stay suspended.
• In-line equalization passes the full flow through a holding basin; off-line diverts only the peak.
• Primary clarifiers settle heavy solids to a sludge blanket and skim floatables off the top.
• Run at least two primary tanks so one can go offline.
• Typical primary removal: 50 to 70 percent TSS, 20 to 40 percent BOD.
• Up to a third of incoming BOD load ends up in primary sludge, pumped on to digestion.
• Thin, watery sludge draw-off means the draw-off rate is too high.
• The settleable fraction, not colloidal or dissolved material, is what a primary clarifier physically removes.
• Coagulant helps fine particles settle in some plants, though most primary clarifiers rely on gravity alone.
• Small SBR and lagoon plants commonly skip primary settling entirely.