9.2 Preliminary Treatment: Screening & Grit

9.2 Preliminary Treatment: Screening & Grit

Quick Answer: Preliminary treatment protects pumps and processes by removing rags, plastics, and grit before primary and secondary units. Bar screens and fine screens capture debris; grit chambers (horizontal, aerated, or vortex) settle heavy inorganics while keeping organics in suspension. Headworks need odor control and strict safety discipline—confined space, H2S, and moving equipment.

Raw wastewater arriving at the plant carries more than dissolved BOD. It brings wipes, rags, plastics, gravel, sand, eggshells, coffee grounds, and storm debris. If those materials enter pumps, clarifier mechanisms, or aeration basins, they clog, abrade, and bury equipment. Preliminary treatment—screening and grit removal—is the plant’s first mechanical defense. On Florida wastewater exams it sits under preliminary/primary treatment process control and safety.

Purpose of Preliminary Treatment

Goals operators should state clearly:

1. Protect downstream equipment (pumps, valves, mixers, diffusers, heat exchangers).
2. Reduce maintenance labor spent pulling rags from impellers and skimmers.
3. Improve primary clarifier performance by removing dense grit that would otherwise fill hoppers and wear scrapers.
4. Improve plant aesthetics and odor at the headworks when screenings and grit are handled promptly.

Preliminary treatment does not replace primary sedimentation or secondary biological treatment. Removals of BOD and TSS across screens/grit alone are modest; the value is mechanical protection and process reliability.

Bar Screens and Fine Screens

Coarse / Bar Screens

Bar screens are parallel bars with openings typically in the ½-inch to 2-inch range (exact opening is design-specific). Wastewater flows through; debris larger than the opening is retained.

As screenings accumulate, headloss rises. Differential level (upstream vs downstream) or a timer typically starts the rake. If the rake fails, the channel can overflow or force flow through a bypass—and bypassing an unscreened channel dumps rags into the plant. Alarms on high differential level are critical.

Fine Screens

Fine screens (openings often from a few millimeters down toward 1 mm or less, depending on type) remove smaller debris after or instead of coarse screening. Types include step screens, band screens, drum screens, and perforated-plate screens. Fine screens are increasingly common where plants want to reduce rags in MBR membranes, UV channels, or fine-bubble diffuser systems.

Tradeoffs:

• Better protection of downstream processes
• More screenings volume and washing/compacting demand
• Greater risk of blinding with grease and fibrous material if cleaning systems fail

Screenings Handling

Captured material is screenings—putrescible, odorous, and a vector attraction. Proper handling includes:

• Washing to return organics to the process flow when equipment allows
• Compacting/dewatering to reduce volume and water content
• Covered storage and timely haul to approved disposal (landfill or other permitted path)
• Never storing wet screenings in open piles that attract pests and generate H2S/odors

Record screenings volume or haul tickets—useful for spotting wipe campaigns or storm debris spikes.

Grit Chambers: Theory

Grit is the heavy inorganic fraction: sand, gravel, cinders, eggshell fragments, and similar high-specific-gravity particles. Organic settleable solids (fecal matter, food particles) also settle, but grit removal aims to capture grit while keeping organics suspended so they continue to primary treatment and biology—not into the grit hopper as a septic, odorous mess.

Design approach: control velocity and detention (or use centrifugal force in vortex units) so particles denser/heavier than organics drop out.

Horizontal-Flow Grit Chambers

Long channels sized so detention and velocity favor grit settling. If velocity is too high, grit carries through and abrades pumps. If velocity is too low, organic solids settle with grit—creating a foul, high-volatile grit that smells, attracts flies, and overloads grit washers.

Aerated Grit Chambers

Diffused air creates a spiral roll. Heavier grit falls to a bottom hopper or trough while lighter organics remain in the circulating liquid. Air rate is a key process knob: too little air → organics settle with grit; too much → grit stays suspended. Surface grease may require skimming.

Vortex Grit Units

Compact footprint units popular in many modern plants. A paddle or tangential inlet creates a vortex; grit settles to a bottom hopper and is pumped to a grit classifier/washer. Operators monitor grit pump cycles, classifier wear, and wash-water quality.

Grit vs Organic Settleable Solids

Exam trap: not everything that settles is grit.

• Grit: primarily inorganic, higher specific gravity, abrasive, relatively low putrescible fraction when well separated.
• Organic settleable solids: food waste, fecal solids—belong in primary sludge or the biological process, not in the grit dumpster.

Poor grit washing leaves a black, odorous product. Good grit is relatively clean sand-like material. Performance is judged by grit quality, equipment wear downstream, and hopper/classifier function—not by maximizing wet tons alone.

Odor Control at Headworks

Headworks are among the most odorous plant areas: turbulent flow strips H2S and other reduced sulfur compounds formed in force mains and long sewers. Florida heat accelerates septicity in collection systems, so coastal and long force-main plants often need:

• Covered channels and wet wells
• Foul-air ducts to biofilters, carbon, chemical scrubbers, or other odor units
• Minimizing free falls and long open channels
• Prompt removal of screenings and grit
• Chemical addition in the collection system (where permitted/practiced) to control sulfides upstream

Odor complaints from neighbors are a plant-management issue; operators control sources by keeping headworks clean, sealed, and ventilated correctly—not by masking alone.

Safety at Preliminary Treatment

Headworks combine confined spaces, toxic/explosive atmospheres (H2S, methane), pathogens, slippery surfaces, moving rakes, and wash-water sprays. Non-negotiables:

• Atmospheric monitoring before entry; follow confined-space permits
• Never defeat interlocks on mechanical screens
• PPE for splash and pathogen exposure
• Lockout/tagout before clearing rags from rakes or pumps
• Respect H2S—deadly at high concentration; odor fatigue is real
• Good hygiene: no eating in process areas; wash after contact

Many operator injuries and near-misses cluster at the headworks. Exam questions often pair equipment knowledge with safe response to a jammed screen or high H2S alarm.

Connecting Preliminary Treatment to the Rest of the Plant

Screenings and grit removal stabilize everything downstream: primary clarifier mechanisms last longer, MLSS systems see fewer rag balls, UV sleeves stay cleaner, and membrane plants avoid catastrophic fiber fouling. When preliminary units are bypassed “temporarily,” document the risk and restore screening ASAP—especially before weekends and storms.

Master screen types and failure modes, grit vs organics separation logic, the three grit chamber styles, headworks odor drivers, and safety. That set owns preliminary-treatment exam items.