PRELIMINARY WASTEWATER TREATMENT

Headworks, "OVERVIEW"
The term "headworks" is usually applied to describe all of the equipment that is at the beginning of the treatment process liquid train. The term is also used to describe the "Preliminary Treatment Process." An example of this is the picture at the left. The picture shows the large, round screw lift pumps (far left side of the picture) supplying raw wastewater to the treatment plant. Once the wastewater is "lifted up" it may then flow pretty much by gravity through the rest of the treatment plant (or at least the next treatment process.) The wastewater first passes through mechanically-cleaned barscreens (top center). Here the large materials are strained out of the wastewater and placed on a conveyor belt for transport. The wastewater then flows thru a grit chamber where the sand, gravel, egg shells, etc are removed from the wastewater. Grit pumps remove the grit from the grit chamber and send it to a grit cyclones/separators (not shown) which remove the damp grit and dump this material on the conveyor belt located on the top of the structure. The conveyor belt then conveys the grit and screenings to a machine which presses the remaining water out. The "dewatered screenings and grit" material is then conveyed into a dumpster that has a plastic bag liner in it. The material is then transported to a nearby landfill. On the top right is the electrical control panel which also houses the SCADA (Supervisory Control and Data Aquisition) panel.

DETAILS

1) SCREENING/RACKING: We first need to remove the very large objects in the wastewater stream, to protect the treatment facility. The rags, cans, sticks, action figures are collected to minimize the clogging of pumps and pipes. This may be accomplished in a variety of ways. (I will discuss a few of them here. ) The most commonly used pieces of equipment are screens or racks. The screens or racks may be cleaned mechanically like that at the left, or manually. Screenings (debris) is most often hauled away or buried. (At the left we see the bar screens depositing the material on the conveyor belt.)

FINE RACKS ARE 1 to 2 INCH OPENINGS, AND YIELD 5 to 10 ft3/ MG; COARSE RACKS ARE 2 to 6 INCH OPENINGS AND YIELD 1 to 3 ft3/MG.

[Note: ft³ means "cubic feet in volume." MG means "million gallons" of water. For example, 1MG is 1,000,000 gallons.]

BARMINUTORS/COMMINUTORS/SHREDDERS (not shown): The rags and trash are cut up and left in the wastewater stream. Some cut pieces may intertwine and hang on primary sedimentation tank equipment, digesters, or wherever downstream. THE OPENINGS ARE usually FROM 1/4" TO 3/4".

OTHER APPROACHES: There are several other approaches, and associated types of equipment used to remove materials from the raw wastewater, like the "Lakeside Rotamat" equipment shown here at the left.)

2) GRIT REMOVAL: Sand, eggshells, cinders, etc., are the next heavier materials that are to be removed from the wastewater process stream. Grit is very abrasive to pumps, settles in other tanks such as digesters where there is no equipment to scrape it off the floor, so it fills the digester up. At the left we have a picture of a non-aerated grit chamber. Aerated grit chambers are usually deep tanks with a slopping bottom that has air injected on one side of the tank to lift and rotate the contents of the grit chamber. The floor is designed to create a "quiet space" where the grit settles out from the rotating wastewater.

In NON-AERATED PROCESSES, the flow is usually reduced to a horizontal velocity in the range of  0.7 to 1.4 ft/Ssec in order for the grit to settle out in the grit chamber. ANY SLOWER, and you also capture ORGANIC SOLIDS, ANY FASTER and the lighter grit washes on through the grit chamber into the next treatment process. The grit may be removed from the chamber by grit screws, scrapers, or conveyors. (The picture to the left shows a non-clog, open-faced impeller, grit pump which pumps the grit from the grit hopper to the cyclone/separator.) In aerated systems the action of the rising air rolls the tank contents moving the grit to the bottom of the tank where it settles out into a hopper. Grit is removed from the hopper by pumps (or gravity/level of the water over it) to be disposed of.

Usually grit (also called detritus tanks) and aerated tanks discharge their grit to cyclones to reduce the volume of water carrying the grit, and include classifiers after that to recycle organics to the headworks and concentrate the grit. (A picture of the cyclone mounted on top of a classifier is shown to the left. The dewatered grit is discharged to the enclosed conveyor belt on the bottom right of the image.)

Sanitary collection systems AVERAGE 1 to 4 ft3. of grit per million gallons (MG); combined collection systems AVERAGE 4 to 15 ft3 per MG. During periods of heavy rain these values may increase dramatically.

3) PRE-AERATION: May follow or precede grit removal. The pre-aeration process freshens the wastewater, aids in oil and grease removal by lifting the oils to the surface, helps remove gases, and aids in coagulation of solids. The normal detention time is 10 to 45 minutes. Diffused air systems normally use 0.5 to 1 ft3 of air per gallon of water treated. Aerated grit chambers may also provide a certain degree of benefit for downstream processes.

FAILURE IMPACT & MITIGATIONS: if you have a rack/screening failure, the debris is bypassed to the next process. If that process is primary sedimentation, then the rags, sticks, etc., will be available to clog sludge pumps, clog the scum withdrawal equipment and pipes. Very light plastics can also carry through to biofilter arm orifices, float in channels, chlorine contact basins, etc. The rags love to hang up on anything...air diffusers, pipes, rake arms, etc. What options are there for you, until the equipment is repaired?

Try installing a temporary rack to screen out the rags and large debris. You will naturally need to manually clean it. Try installing screens on your primary sedimentation tank launders to catch the neutrally-buoyant carry-through plastics and debris. If the grit removal system is bypassed: the grit, in very large quantities, will cause premature wear on the primary sedimentation process flights, pumps, etc. Grit can clog sludge pumps, plug the sludge pipes, and load up the sludge removal equipment. To help reduce grit impacts for the very short term solution: you might be able to slightly increase the amount of sludge pumping to reduce the heavier solids content to some extent, but you must be careful not to upset your digesters by diluting your solids too much, also causing more energy to heat the digesters, filling digesters with grit, and washing out your buffering capacity.  The higher solids in your supernatant recycle to your headworks which will increase your organic loading. (Obviously not something I would try first!) You might be far better off trying to install a portable pump to convey your grit (if your pumps are out of service). Try pumping your grit to a garbage dumpster to settle out, with the excess water returned to the wastewater stream, if your cyclone or classifier is out of service. If your pre-aeration blowers are out of service, consider a portable air compressor(s) to provide air if it is necessary to keep the tanks in service. You'd be amazed what you can do at this point with a little "fantasy thought" and some rental equipment! Never give up!

Return to Past Operator Notebook List

Return to Wright's Training Homepage