Chapter 3

At a minimum, how often should screens be inspected for visible and audible indications of possible malfunctions?

(A) At a minimum, screens should be inspected daily for visible and audible indications of possible malfunctions. As a routine practice, the operator needs to observe all moving mechanisms to determine if the components are free of obstructions, properly aligned, moving at constant speeds, and producing no unusual vibrations. The operator should periodically verify that all control lamps are functional and that there are no burned-out light bulbs. The operator must listen to all moving mechanisms and recognize their normal operational sounds. For example, abnormal screeches could indicate lack of lubrication; thumps could indicate broken or loose components.

This type of screen has the widest openings and only stops the largest debris.

(A) Coarse screens or trash racks have the largest openings. Debris removed from screens with wide openings typically consists of wood, tree limbs, rocks, and other large items.

The lower port of a grit cyclone is partially blocked by a piece of trash. The blockage will likely cause

(A) Excessive vibration of a grit cyclone may be caused by an obstruction in either the upper or lower port.

In an aerated grit basin, this process control parameter influences which particles will sink and which will continue on to the next process.

(A) In an aerated grit basin, airflow causes a spiral roll pattern perpendicular to the flow through the tank. The velocity (speed) of the roll pattern influences which particles will drop to the bottom. The particles that remain will be carried into the tank effluent. The velocity of the roll is governed by the shape of the grit basin and the amount of air introduced. Roll velocity is independent of flow through the tank, which allows this type of grit basin to operate effectively over a wide range of flows.

What is the most likely effect of receiving large quantities of grease on a perforated plate-type screen?

(A) Perforated plate improves capture of hair and other fine material that can pass through bar screens and wedge wire, but it is also more likely to become clogged or "blind off."

A pond WRRF is most likely to have this type of screen in its headworks.

(A) Ponds are tolerant of debris and grit and tend to have screens with much larger openings at their headworks.

Gravity pulls grit particles straight down, but this pushes them forward toward the outlet of the grit basin:

(A) When a particle enters the grit basin, gravity pulls it down in a straight line. At the same time, the water moving through the grit basin is pushing the particle forward. How fast the particles move forward is related to the velocity of the water. The result is that most of the particles entering the grit basin will have a diagonal path from the front of the basin toward the end of the basin.

This device may be used to direct flow to or around a screen or grit basin.

(B) A slide gate is placed within a channel to direct flow. The gates may be used to divert flow and allow a particular piece of equipment to be taken out of service for inspection, cleaning, and maintenance while leaving other pieces of equipment in service.

An operator is used to seeing an even mixing pattern on the surface of the aerated grit basin. Today, the surface is fairly smooth and does not show signs of good mixing. The operator should

(B) Diffuser (device releasing air into the wastewater) openings should be cleared regularly. If surface turbulence diminishes, the diffusers may need cleaning to remove rags or grit.

An operator must bypass their aerated grit basin for several days while annual maintenance is performed on the diffusers. Which of the following statements is true?

(B) Grit removal protects downstream equipment from abrasion and accompanying wear.

A sample of grit is grayish in color and smells like rotten eggs. The operator should consider

(B) Grit that is grey, greasy, or has a foul, rotten egg odor indicates the presence of excessive amounts of organic material. This indicates that flow velocities are too low or that some other adjustment needs to be made to either the grit removal or grit washing process.

This type of control mechanism is best suited for storm events or other conditions that produce highly variable amounts of screenable material.

(B) Timers work well but cannot adjust to accommodate sudden large accumulations of debris that can occur during high flow periods, such as a storm event. Level-sensing devices are typically installed in parallel with the timers and are designed to override the timer control. Level-sensing devices measure the water height in the channel immediately upstream of a screen. If the water level is rising, it means material is accumulating on the screen and head loss is increasing.

Screens should be cleaned before the head loss across the screen reaches ______ or according to the screen manufacturer's recommendations.

(B) Unless otherwise specified by the manufacturer or the WRRF's operations and maintenance manual, at a minimum, screens should be cleaned when the head loss across the screen reaches 7.6 cm (3 in.) (Bristow, 2008).

A grit basin has a flow velocity of 0.3 m/s (1 ft/sec) at 9 a.m. As flows increase throughout the day, grit basin performance will be affected in this way.

(B) Velocity is equal to the amount of flow divided by the cross-sectional area of the channel, pipe, or basin. When flow increases and area stays the same, velocity will increase. As flow velocity through the basin increases, larger, denser particles will pass through to the next process, decreasing grit removal efficiency.

Septage wastes received by WRRFs should be monitored and tracked to

(B) When accepting septage, operators should monitor and track incoming loads and calculate the total mass of BOD, TSS, and ammonia-nitrogen received (kilograms [pounds]). Tracking the total mass received will help to ensure that the amount of septage received does not exceed the WRRF's rated treatment capacity.

The headworks building at a new WRRF shows evidence of metal and concrete corrosion. The problem seems to be even worse in covered channels. The most likely cause is

(C) Adequate ventilation is essential if a grit basin is enclosed. Otherwise, the corrosive atmosphere will inevitably affect exposed electrical wiring and controls and concrete, as well as the risers and headers.

One disadvantage of using comminutors and grinders is

(C) Comminutors and macerators grind or chop solids that could interfere with downstream wastewater treatment processes. These devices do not remove the chopped solids from the wastewater flow. Comminutors increase the amount of inert material that will accumulate in the aeration basins and digesters, which effectively decreases treatment capacity.

A septage hauling and receipt manifest should include all of the following information EXCEPT

(C) Each septage load received should have a tracking manifest that includes wastewater characterization information, the generator's information and certification statement, and a hauler's section. The hauler's section includes the septage receipt date and vehicle license number but not the commercial driver's license number.

A WRRF currently has a bar screen with 50-mm (2-in.) openings. Operators of the WRRF are considering replacing the screen with one that has 25-mm (1 in.) openings. How much should they expect the volume of screenings removed to change?

(C) For screen openings between 25 and 50 mm (1 and 2 in.), for each 13-mm (0.5-in.) reduction of clear opening size, the volume of screenings will approximately double. In other words, a screen with 25-mm (1-in.) openings will remove about four times the amount of material as a screen with 50-mm (2-in.) openings.

One consequence of allowing excess organic material to be removed along with rags and other inert debris is

(C) Screened material that is not washed sufficiently may retain organic material that can putrefy and generate odors and hazardous gases.

This type of grit basin may use paddles to maintain the flow circulation pattern.

(C) Some vortex tank designs rely on natural hydraulics to achieve the proper rotational rate, whereas other designs use a slow, rotating-paddle-type mixer to enhance the natural hydraulics and achieve proper separation.

A WRRF currently has one grit basin in service. The flow velocity through the basin is 0.8 m/s (2.6 ft/sec). How many grit basins should be in service to maintain a flow velocity between 0.24 and 0.3 m/s (0.8 and 1.0 ft/sec)?

(C) When flow increases and area stays the same, velocity will increase. When flow is changing, then area must also change to keep velocity constant. To solve this problem, divide the current velocity by the desired velocity. The current velocity is 0.8 m/s (2.6 ft/s). The maximum desired velocity is 0.3 m/s (1 ft/s) 0.8 ÷ 0.3 = 2.7 basins in service The minimum desired velocity is 0.24 m/s (0.8 ft/s). 0.8 ÷0.24 = 3.3 basins in service

All of the following characteristics will influence the settling velocity of a particle EXCEPT

(C) color. Settling velocity (i.e., how fast a particle sinks) depends on the density of the particle as well as its shape and size.

Grit basins typically remove sand, gravel, eggshells, and coffee grounds by

(D) Grit basins are channels or small tanks where the wastewater velocity, or speed, is decreased to about 0.3 m/sec (1 ft/sec). Here, heavier particles such as sand, grit, eggshells, and heavier organic particles settle to the bottom of the tank where they can then be removed.

Which of the following particles should settle the fastest through a column of water?

(D) How fast a particle sinks or floats (settling velocity) is related to how different the density of the particle is from the density of water. A particle that is only slightly more dense than water will sink slowly, whereas particles that have high densities will sink rapidly. Round, compact solids sink faster than flat, feathery particles. Larger particles sink faster than smaller particles made of the same material.

On a single rake screen, a limit switch

(D) In the unlikely event that the rake becomes jammed, a limit switch is activated to turn off the drive motor.

This test measures the amount of free water in screenings and grit:

(D) Screenings and grit must, at a minimum, pass the paint-filter liquids test before they can be accepted for disposal at a landfill. This test measures the amount of free water that leaches from the screenings by measuring the quantity of liquid from a representative screening sample that passes through a filter of Mesh No. 60. After the 5-minute test period, if any liquid passes through the filter, the material is deemed to contain free liquid.

Grit is typically removed from a vortex-type grit basin in this way.

(D) Vortex grit chambers swirl the raw wastewater in the chamber using gravity. The velocity of the vertical roll pattern (whirlpool) allows denser particles to settle while keeping lighter organic material suspended. The grit that settles in these tanks can be removed by an airlift pump or a non-clogging, recessed-impeller-type grit pump.

Flow-proportional weirs are used in velocity control horizontal flow grit basins for this purpose:

(D) When flow increases and area stays the same, velocity will increase. When flow is changing, then area must also change to keep velocity constant. Horizontal flow grit chambers use proportional weirs or rectangular control sections to vary the depth of flow and keep the velocity of the flow stream constant.

Fine screens capture fewer organic solids than coarse screens.

(F) Fine screens capture more organic solids than coarse screens, so operators should transfer them more frequently to final disposal, before excessive odors are produced.

Mechanical cleaning of bar screens increases labor costs because of increased maintenance.

(F) Mechanical (as opposed to manual) cleaning reduces labor cost, improves flow conditions and screening capture, and reduces nuisances.

A WRRF with a combined sewer system is expecting a large storm. The manual bar screens are normally cleaned every hour. The operator should plan to decrease the cleaning frequency during and immediately after the storm.

(F) Screenings should be removed from the face of the screen as often as necessary to ensure a reasonably free flow of wastewater. During rainstorms, the cleaning frequency may have to be increased. In installations with multiple channels, additional screens may be placed into service during a storm event.

Because septic waste is already partially treated, it tends to be lower strength than typical domestic wastewater.

(F) Septic waste is domestic wastewater collected from septic tanks, cesspools, portable toilets, pit toilets, and recreational vehicles. Septage tends to be much higher strength than typical domestic wastewater. Biochemical oxygen demand (BOD) is typically 26 times more concentrated compared to typical raw domestic wastewater and may be as much as 300 times more concentrated (WEF, 1997).

The volume of grit removed for every m3 (mgd) of flow received is very consistent from one facility to the next.

(F) The amount of grit removed is different from one WRRF to another. In a 2008 WEF member survey, grit quantities reported by WRRFs ranged from 3.7 to 148 L/1000 m3 (0.5 to 20 cu ft/mil. gal) and averaged 37 L/1000 m3 (5.0 cu ft/mil. gal). Variables influencing the quantity of grit include the type of collection system (combined or separate), presence of household garbage disposals, condition of the collection system, presence and types of industrial waste, and efficiency of grit removal.

All WRRF headworks include screening, grit removal, and flow measurement.

(F) The goal of preliminary treatment is to reduce the chance of damage to downstream equipment or processes and clogging of pipes. Preliminary treatment takes place at the WRRF headworks and typically includes screening and grit removal. The headworks may also include flow measurement. However, a particular WRRF headworks may have all, some, or none of these processes.

When grit pumps are used to remove grit from a hopper, a timed cycle of 10 minutes of operation once every 30 minutes should always be used.

(F) The pump should cycle frequently enough to prevent the grit from compacting in the hopper. The cycle length, or total minutes of pump time, should be coordinated with the hopper size. The time between pumping cycles is determined by how long it takes for the hopper to fill with grit. Operators will need to observe conditions at their facilities to determine the optimum pump cycle and run time.

Screening and grit removal must be completed before flow measurement.

(F) Unit processes in a WRRF headworks can vary and so can the order in which they are arranged. Flow measurement may come before or after screening and grit removal.

Velocity is calculated by multiplying the flow rate by the cross-sectional area.

(F) Velocity is equal to the amount of flow divided by the cross-sectional area of the channel, pipe, or basin.

Grease trap waste can be accepted and treated by most WRRFs under the 503 Regulations as long as it comes from restaurants and not categorical industrial users.

(F) WRRFs can accept septage from domestic sources, which are regulated under the Code of Federal Regulations at 40 CFR Part 503, but should not take grease trap waste or industrial septage because this is regulated differently than domestic wastewater. Nondomestic septage, such as grease trap waste, is regulated under 40 CFR Part 257.

When placing a rectangular grit basin with a chain-and-flight system into service, the basin should be filled completely before starting the mechanism.

(F) When initially filling grit tanks equipped with collection mechanisms, the influent gate should be opened slowly. Once the collection mechanism is submerged, the operator may then turn on the longitudinal collectors (flights), the screw collector (if any), bucket elevator or grit pump, and, if present, the grit washer and classifier. This approach avoids startup under load conditions after the grit builds up.

The water level in flow equalization basins should fluctuate over a 24-hour period.

(T) Flow equalization basins temporarily store influent wastewater during peak hour flow and storm events. The water level in the tank changes over the course of a day, filling up during peak hour flows and emptying again during periods of low flow. Ideally, the flowrate out of a flow equalization basin and into the WRRF will be at a constant rate.

Grit classifiers return organic material to the wastewater flow and remove excess water from the grit.

(T) Grit classifiers can effectively remove organic material from grit, returning it to the wastewater flow, while dewatering the grit.

As flow velocity through the basin increases, larger, denser particles will pass through to the next process.

(T) Grit removal systems rely on differences in settling velocities to remove grit while allowing organic solids to pass on to the next treatment process. However, if the flow velocity through the grit basin is too high, even very dense particles will be pushed through to the next unit process. If the velocity is too low, excessive amounts of organic material will be removed with the grit.

Increased equipment run time will increase O&M costs.

(T) Grit removal systems start at preset times and operate for preset times. Increased equipment run time will increase O&M costs. The operator must consider wear of grit removal equipment vs increased removal efficiency and resultant downstream equipment longevity.

Head loss is the difference in water level between the upstream and downstream sides of a screen.

(T) Head loss is the difference in water level between the upstream side of the screen and the downstream side of the screen. As material accumulates on the screen, it becomes more difficult for water to pass through the screen. If the screen is not cleaned often enough, water builds up behind the screen and can overflow the channel. The smaller the spacing, the more debris is captured, with greater head loss developing across the screen.

If too much organic material is removed at the WRRF headworks, the downstream biological process can be affected.

(T) Removal of too much organic material with the screenings can impede the ability of some biological treatment systems to remove nitrate. As screen openings become smaller, greater amounts of organic material are removed, and it becomes important to wash and compact the screened material and return organic material to the wastewater flow.

Rollback occurs when screened material falls back down the face of the screen, possibly being returned to the wastewater flow.

(T) Rollback is when screenings fall or roll off the front of the screen instead of being carried up and out of the flow path.

Septage can contain rocks and other debris, which can damage downstream equipment if it is not screened out.

(T) Septage may contain higher concentrations of FOG, heavy metals, toxic organics, trash, rocks, and debris.

A shear pin is designed to break under a certain amount of force or pressure to prevent additional damage to equipment.

(T) Shear pins are used in many different types of equipment to prevent an excessive buildup of force or pressure. For example, if a piece of debris becomes jammed in a screen and the screen is unable to move, the motor will continue to operate and push the screen. Excess force can build up, potentially damaging the screen. The shear pin is designed to break before equipment is damaged.

Removal of grit, rags, and other debris is often necessary to protect downstream equipment from harm.

(T) The passage of rags and debris into downstream processes is one of the biggest causes of equipment maintenance and failure because of jammed pump impellers, clogged sludge and scum pipelines, and imbalanced operation of rotating equipment. Floating material in downstream processes is unsightly and can be a safety hazard to operators attempting removal. Screens are typically a WRRF's first treatment unit and protect facility equipment against damage. The type of screen selected will depend on which treatment processes are downstream and how much protection they require.

Trash and bar racks are installed at a 30- to 45-deg angle from the vertical.

(T) Trash racks, also called bar racks, are constructed of heavy, parallel rectangular or round steel bars with wide spacing set in a channel. The bars are sloped at an angle ranging from 30 to 45 deg from the vertical.

A bar screen may not prevent this type of material from entering the WRRF.

A bar screen with wide openings will stop large debris from entering the WRRF, but rags and stringy material may still pass through.

Select a screen type that meets the needs of the downstream process.

The downstream process is sensitive to rags and stringy material. The continuous element, perforated plate screen performs best at removing stringy material.