Wastewater Treatment: Pretreatment and Primary Sedimentation
Thick Grinding Lattices
At this stage, there are usually several lines. For the process, thick bars are installed with a clearance between plates of 30-50 mm and below (downstream) a fine grating with a clearance from 15 to 20 mm. The function of these bars is not to remove the sand but rather the solid minerals. When minerals do not provide solid, specific discharge (straw, hay, etc.), a finer grinding is required, called refining sieve or fine mesh of about 3-6 mm.
The forms in which the grids can be:
- Curved grids: Do not give good results for “dizzying” the retained solids.
- Vertical grilles: At an angle to the channel of 75-80º.
To remove the solids retained by the grid and prevent blockage, a comb is used that provides co-current flow if the comb attacks from upstream or countercurrent if it attacks from downstream.
Points to consider:
- The speed of approach in the channel must not be less than 0.3 m/s.
- Always place a bypass.
- Consider maximum speeds passing through the grid.
- Consider the usable area.
- If it is not explicitly stated, consider a percentage of binding.
Grit Chambers and Degreasers
Grit chambers can be static, dynamic, and aerated. Aerated grit chambers, such as those conceived by Kalbskops, will be discussed. They have two compartments: a larger main section which is the area of sand removal and another smaller side section which is the degreaser. In the first, air is injected through a perforated tube fed through various channels. Air provides a vertical component to the movement of wastewater. It is necessary to graduate this injection of air and water velocity in the helical flow of the grit chamber to not re-suspend the sand particles but to keep organic solids in suspension. An additional effect of the injected air is to break down the emulsified fats. A screen is intended to calm the flow in the fat area. The bottom of the sides of the sand trap will have a slope close to 45 degrees, which is optimal. It also provides a bridge that moves longitudinally over the sand trap that holds the blades. On the other hand, sand is extracted from the hopper. The system used is usually a pump placed on the mobile bridge that sucks the flow of sand through a flexible tube. This avoids clogging of sands during storms. The drive (a mixture of sand) is sent to a side channel with a longitudinal slope, leading the sand to a washer-sorter device. Excess water runs off and washes the sand from the bottom. The output of the sand trap is through a deflector landfill which ensures the constant retention volume and prevents the output of floating materials.
Primary Sedimentation
The purpose of primary sedimentation is to eliminate 95% of settleable suspended solids (which settle in approximately one hour). 60% of these solids are organic matter. There are two types of clarifiers: rectangular and circular. The rectangular settling tanks are similar to the grit chambers but with a longer retention time. They are fed by a distribution channel. A movable bridge carries a sweeping sediment scraper that pivots, moves counter-currently, and floats back up to a “bib” (a small black screen). The circular clarifiers can have a deflector cylinder or a lower output. It is often more common to use the lower output.
The design parameters are:
- Surface loading rate
- Retention time
Values:
- 1.3 – 2.5 m3/m2/h at Qmed
- 1.3 – 2.5 m3/m2/h at Qmax
This process yields:
- SS = 60-70%
- BOD5 = 30-35%
The reduction of BOD5 is sought to be of the order of 30%, and the speed is thus limited. The reduction of SS is around 65% as the fraction above 70% is no longer settleable and cannot be eliminated. The retention time (RT) is at least 1 hour at Qmax and 2 hours at Qmed. The slope of the sill is between 1/10 and 1/15. DIN standards relate the diameters of the well, hopper, and clarifier. The flow unit on the landfill (dump load) must be ≤ 40 m3/h/ml. The spillway channel has a minimum size to be formed and not given pending. The purge of sludge will never be less than 150 mm in diameter.
In the primary settling, we have the following input/output concentrations:
- BOD 300 mg/l, SS 300 mg/l => BOD 200 mg/l, SS 100 mg/l
These levels of output from the primary sedimentation cannot be reduced further mechanically. They are present in dissolved or colloidal suspension. To reduce the SS and BOD5 not eliminated in the previous processes, there are two procedures: biological and chemical.
Pretreatment: Thick Well Operation
In a treatment plant, a thick well is essential. In desert climates, there is strong erosion which results in the shifting of sands. The purpose of the well is to remove the thick materials causing impacts, abrasions, jams, and damage to equipment. The well works by decanting and has a lateral spillway and a gate at the output.
For the thick well design, only two parameters are taken into account:
- The retention time (RT)
- The hydraulic loading rate or surface overflow rate (SOR)
Retention time (RT) should in no case be less than one minute, referring to the allowable flow of the plant, and never less than two minutes if we refer to the peak flow (Qp) of sewage. The surface overflow rate (SOR) will be less than 600 m3/m2/h if we take into account the allowable flow rate (Qa), and less than 300 m3/m2/h for peak flow (Qp). The thick well has a trunk-pyramidal recess to store gravel and gravel stockpiles.
The retention time (RT) and surface overflow rate (SOR) determine the effective height of the thick well. The deposits are removed with a hoist hanging on a grapple, generally 0.75 m in width, which determines that the bottom of the well should not exceed 1 meter wide so that the bucket can coat the bottom. The base is chamfered at an angle that should be greater than 45 degrees around the perimeter.
The collection area (useless) has a depth of calculation but is conditioned by the geometry of the well, usually about 1 to 1.5 meters. The bottom and side slopes of the well are protected, as well as rail profiles, to avoid the impact of the bucket with concrete.
The air flow in the well will be 0.2 Nm3/m3 of bulk water to be treated, calculated for average flow. The grid is robust and has a clearance between bars of 80-150 mm. When it is cleaned, the exterior is cleaned, and during this process, solids slip on the ground. The thick shaft is provided with a spillway. Although the arrival collector can have one, for safety, an overflow is placed in the well. In general, throughout the plant, an overflow must always be provided at points where there is a gate (possible obstruction). The lip of the weir is placed at the height reached by the water level in the collector with a permitted flow equal to Qa. The water level should never exceed the top of the unit, so this should be above the height of the last overflow of the collector. In horizontal topographies, and to avoid digging, knowing that a plant has about 3 or 4 meters of head loss, a pump is needed.
Pumping in Wastewater Treatment
The water at this point in the water line is highly charged with solids that may cause binding and strong abrasion of pipes and pumps. Types of pumps:
Centrifugal pumps:
a) Closed impeller: These have a limited step. They easily get stuck with raw water and sludge, therefore they are not commonly used.
b) Vortex impeller: The flow is axial. It has only a plate or disk and fans the water to produce a vortex. They are eligible but have low yields.
c) Single-channel impeller, or 2 or 3 channels: They have a very open one turn or several turns for different channels. It has a larger step than the closed impeller, making them eligible for dirty water or mud, with somewhat greater performance than the vortex.
Non-centrifugal pumps:
Archimedes screw: A large pump. From the filling point, the flow is steady, but the level of the filling precinct swings a lot (making sure that the level does not fall below the filling point). No variation of high flow is regulated as well. It is a very robust structure and very difficult to jam. The civil work is long and complicated as it requires angles from 30 to 35 degrees maximum, a round bed, building engines, etc. The low yield curve varies very little if the layer is lowered below the filling point, saving energy. Civil works are more expensive, but such pumps are preferable.
Submersible: Along with the screw, these are the most commonly used in raw water. With these pumps, the civil works that would be required for a dry pump are avoided. There may be very strong oscillations of the flow that reaches the plant. It would be desirable to work only with a minimum flow and a higher flow pump, the greater the number of them, but the number of pumps installed makes the operation much more expensive. The flow rate may vary between 0.5 and 10 x Qmed. The size of the pump should be such as to continuously raise the Qmin, or at least, to spend two or three times an hour.