Wastewater treatment plant ( Grit chamber , Primary sedimentation tank, Biological treatment), Assignments of Civil Engineering

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University of Wasit ﺟﺎﻣﻌﺔ واﺳﻂ College of Engineering ﻛﻠﯿﺔ اﻟﻬﻨﺪﺳﺔ Department of civil engineering ﻗﺴﻢ اﻟﻬﻨﺪﺳﺔ اﻟﻤﺪﻧﯿﺔ

Report on (Wastewater treatment plant ( Grit

chamber , Primary sedimentation tank, Biological

treatment)

Prepared by :

Shahad Salim Qata’

Supervised By Asst.Prof. Ali Jwaid

Contant page

1. Introduction 3 1.2 Wastewater Basics 3 1.3 Biochemical Oxygen Demand (BOD)

1.4 Chemical Oxygen Demand (COD)

2 Primary Treatment 6 2.2 Grit Chambers 8 2.3 Primary Sedimentation 9 2.4 Biological treatment 11 Calculation And References 12

1. Introduction 1.1 Background Water is necessary for life on earth. In nature it comes in three aggregate forms (snow / ice, liquid, vapour / steam). The majority of particle transport is carried out through water. In water many particles become dissolved. Water also has a great capacity to take on heat. In our society water fulfills many functions as the foundation for a natural habitat and more specifically for human activities, such as domestic use, industrial and agricultural usages, fishing, transportation and recreation. A byproduct of human usage is that the water then becomes polluted with contaminants. As a result of this the water quality in the various compartments of the water cycle (ground water, surface water) is influenced. One of the most succinct examples of a disturbed water cycle is shown through the discharge of raw household and industrial wastewater into the surface water. In many cases the natural storage capacity limited to such a degree that a disturbance of the natural functions occur 1.2 Wastewater Basics 1.2.1 Water Basics

• H2O • dipole • solid, liquid, gas (0°C, 100°C) • density (1,000 kg/m3) • heat: speci c heat 4.18 kj/(kg ˚C); heat of evaporation/enthalpy of vaporization 2, kj/kg • viscosity: 1.0 mPa.s with 20˚ • contaminants: dissolved (< 10 nm), colloidal (10 nm-1 μm), suspended solids (> 1 μm) • water as a solvent: gases (Henry’s Law), liquids (miscibility) • ionisation: ions, acids, bases • oxidation-reduction • biology: bacteria, pathogens, substrate, nutrients

1.2.2 Wastewater Sources Wastewater that is treated in a municipal wastewater treatment plant (wwtp) or sewage treatment plant (stp) can have various sources. The influence of the typical wwtp consists mostly of wastewater from households and businesses (urban/municipal wastewater, or “sewage”). There may also be industrial wastewater discharges to the wwtp. Often large industry and industry parks will pretreat their own wastewater in a special wwtp before it is discharged to the municipal sewer or main wwtp. Water is used with many industrial processes and wastewater is produced. The amount and composition of wastewater are strongly related to the type of industry. And even the degree of internal recirculation and water saving has a large influence. If the industrial treatment is adequate, it may be allowed to discharge directly to the surface water. Wastewater that comes from pure municipal and urban sources is of a fairly constant composition. If industries are also connected to The wwtp, the influent wastewater composition varies depending on the type of industry and the management. The contaminants transported by these wastewater flows determine the (biological) capacity of the wwtp required to treat it. In addition to the municipal and industrial sources of wastewater that make up the base component of a ow to a treatment plant (dry weather ow), the drainage of rainwater to the wwtp (storm water ow) will also contributes to the total wastewater  ow to be treated at the wwtp. The extent to which stormwater will impact the wwtp is related to the type of collection system in place, as combined systems will carry much more stormwater to the wwtp than seperate systems. This storm water flow determines to a large degree the required hydraulic capacity of the wwtp.

1.3 Biochemical Oxygen Demand (BOD) The biochemical oxygen demand, BOD, is the amount of oxygen in mg that is necessary to transform biochemical oxidizable elements by means of bacteria present in 1 litre of water. To measure, mix a sample of wastewater with pure water with an oxygen content that is known and determined, after the mixture has been stored (usually) 5 days in a dark place at 20˚C, how much oxygen is used for the oxidation of the organic matter. The test must take place in the dark in order to prevent the occurrence of oxygen producing algae from happening simultaneously. There are two oxygen measurements necessary, namely one at the onset and one at the end of the experiment. 1.4 Chemical Oxygen Demand (COD) In the determination of the COD most organic compounds are nearly all chemically oxidised. Potassium dichromate is used as an oxidizing agent. In determining the COD the following is added in a sample: • A known amount of potassium dichromate (K2Cr2O7); • A certain amount to silver sulphate (Ag2SO4) that serves as a catalyst agent for the oxidation; • Mercury(II) sulphate (HgSO4) to prevent chloride from oxidizing. The remaining amount of potassium dichromate is determined after two hours of boiling under back flow cooling in a basket. The consumed amount of oxygen is calculated by comparing the difference between the original amount of potassium dichromate and the remains. The results obtained are much less susceptible to fluctuations than the results from a BOD determination.

2 Primary Treatment 2.1 Screens 2.1.1 General Wastewater carries certain solid materials such as wood, plastic and  brous material. These substances can cause serious problems in the treatment process by plugging the pumps and mains or by forming  coating layers in the digestion tanks. The size is usually such that it is possible to remove it through a screen or sieve. The placement in the process is usually directly after the influent pump. 2.1.2 Bar Screens Bar screens are made up of a number of parallel rods that are equidistant and are positioned at an angle of circa 75˚. Cleaning the screens is carried out by a raked bar screen. This can be done manually as well as automatically. The screen waste, also known as screenings, is often removed through a conveyor belt to a container or to a screenings press. Sometimes a reduction is adjusted ahead of time. Often a bypass pipe is installed at the screen which allows the water to drain if the screen is clogged.

2.1.3 Continuous Screen Presently continuous screens are often being installed where the screen moves through the sewage and takes with it the waste; the waste can also be removed from the sewage by a slow combined movement of the screen components. Especially for these types very scant openings (3-8 mm) are installed 2.1.4 Sieves Sieves are installed in the treatment of wastewater as  ne screens. There are various types in use: One of these is the drum sieve. This sieve is used for example for chicken slaughterhouses to remove the feathers and offal. A drum sieve consists of a slow rotating drum that is equipped with small perforations. The drum is driven through a gear box by an electric motor. The wastewater to be treated is brought inside the drum and pushed back out through the perforated casing. The sieved particles stay in the drum and through both the revolving movement of the drum and the internal screw are placed at the end of the sieve screen and then cast out. More small particles from industrial wastewater can be removed from wastewater by sleves than by installing screens

2.2 Grit Chambers 2.2.1 General There are various reasons why sand and grit must be removed from wastewater:

• to extend the lifespan of the mechanical components, especially pumps;
  • to prevent sand and grit from getting into the pipelines and machinery, which can cause blockages.
• to avoid depositing a sand package at the bottom of the digestion tank, the presence of which would minimize the effective volume and hence the efficiency of the tank.
  • In a grit chamber one tries, through selective sedimentation, to remove grit and similar mineral material with a grain diameter of > 0.15 mm; The placement of a grit chamber is preferably completely at the beginning of the purification process. When it is installed in an influent pumping station, the grit chamber is, for technical and practical reasons in general situated, on the surface and often after the screens.In a plant with sludge digestion grit chamber can be placed in the sludge line, for example between the primary treatment and the sludge thickening or between the sludge thickening and the sludge digestion. As a result of this one can save costs on the size of the tank; in fact the amount of sludge that needs to be treated is noticeably less than the total wastewater ow 2.2.2 Dimensioning : Sand particles behave as discrete particles during sedimentation. By setting the surface load (vo) of the grit chambers equal to the settling velocity of the sand particles (vb) a successful sedimentation is realized. Q/A is also called surface load; the depth only plays a limited role.

2.3 Primary Sedimentation 2.3.1 Introduction Usually the primary sedimentation tank comes after the grit chamber. Here as many of the settleable undissolved particles as possible are separated. This sludge is called primary sludge. In some plants (oxidation ditch types) where there is no primary sedimentation tank installed, the undissolved particles are caught in the activated sludge and are stabilized there. 2.3.2 Sedimentation in Practice Different varying factors influence the sedimentation process such as those that occur in practice. The ef ciency of the sedimentation depends on:

• the size and the form of the particles; the larger the particle size, the faster sedimentation occurs;
• the density of the particles; if the difference between the particle density and the carrier liquid is larger, then the sedimentation process is faster; • the composition of the suspension;
• the concentration of the suspension; the larger the concentration , the larger the sedimentation process efficiency.
• the suitability of the particles to occultation;
• the temperature; with higher temperatures, the viscosity of the liquids is reduced and thus particles settle faster;

2.3.3 Design Aspects : The most important design criteria for sedimentation tanks is the set surface load. In general 1.5 – 2.5 m3/(m2·h) is assumed as for maximum load. If this maximum load has little or no effect on the in uent  ow pattern, then higher values (up to 4.0 m3/(m2·h)) are used. As a residence time a minimum value of 1 to 1.5 per hour is kept. In terms of the minimum and maximum tank sizes the following is recommended: • round tanks diameter minimum 20 m maximum 60 m optimal 30-40 m with smaller tanks (D<30 m) the efciency is reduced due to interference by inuent ow and discharge depth 1.5 - 2.5 m bottom slope 1:10 to 1:12. • rectangular tanks length maximum 90 m optimal 30-50 m width 5-12 m usually 5 à 6 m depth 1.5 - 2.5 m bottom slope 1:10 to 1: 2.3.4 Round Tanks: With round sedimentation tanks the wastewater is fed into the middle and is discharged through a trough on the outer periphery. Above the tank there is a bridge that slowly rotates with sludge scrapers attached, these move the sludge on the bottom slowly towards the central sludge funnel, where it is removed. The bottom is built with a slight slope. For the efuent trough there is a scumboard or baf e for holding back the  oating solids. The  oating solids (usually fat) are pushed into a scum trough by the  oating scum scrapers that are attached to the bridge. 2.3.5 Rectangular Tanks: Rectangular sedimentation installations have a basin with flat or slightly sloping bottoms, equipped with an incoming and outgoing construction. In addition a scumboard is installed to keep back the floating scum

2.4 Biological treatment a biological wastewater treatment system is a technology that primarily uses bacteria, some protozoa, and possibly other specialty microbes to clean water. When these microorganisms break down organic pollutants for food, they stick together, which creates a flocculation effect allowing the organic matter to settle out of the solution. This produces an easier-to-manage sludge, which is then dewatered and disposed of as solid waste. Typically broken out into three main categories, biological wastewater treatment can be:

1. aerobic, when microorganisms require oxygen to break down organic matter to carbon dioxide and microbial biomass
2. anaerobic, when microorganisms do not require oxygen to break down organic matter, often forming methane, carbon dioxide, and excess biomass
3. anoxic, when microorganisms use other molecules than oxygen for growth, such as for the removal of sulfate, nitrate, nitrite, selenate, and selenite Depending on the chemical makeup of the wastewater in relation to the effluent requirements, a biological wastewater treatment system might be composed of several different processes and numerous types of microorganisms. They will also require specific operational procedures that will vary depending on the environment needed to keep biomass growth rates optimal for the specific microbial populations. For example, it often is required to monitor and adjust aeration to maintain a consistent dissolved oxygen level to keep the system’s bacteria multiplying at the appropriate rate to meet discharge requirements.

Calculation Q/ Design a grit chamber and primary sedimentation tanks for wastewater treatment plant served population 1250000 capita and q= 280liters / capita .day , if BOD inlet = 380 mg/l and SS inlet 400 mg/l. DWF = Qave = Nq = 1250000280 = 350000 m3/day 3WF = 3* 350000 = 1050000 = 729.16 m3/min Use t = 2min, (2-5) min. Volume V = 729.16 * 2= 1,450.33 m Using at least two tanks = V = = 729.16 m 2 1,450. Let the depth of the tank H = 4m and L = 3w Area = Volume/Depth = 729.16 / 4 = 182.3 m Area = WL ⇒ 182.3 = 3 W^2 ⇒ W = 8m L = 3 W ⇒ 38 = 24 m Dimension of the chamber ( LWH) = 248 Checking dimension time ( should be between 2-5 min.) T = V/Q = = 2.10 minutes OK. 2

24 8 4* *

Designing of power supply (Blower)

Qair = (0.3 − 0.7) = 0.5 * 24 = 12 m3/ min

m 3 min − m length

P = [( ) -1 ]

8.41 (^) * E M (^) * R T * P 1 P (^2) γ γ− Air density = 1.2 kg/m Mass = density × volume Mass = ρ × volume Mass = ρ × Q × t = 1.2 * 12 * 1min/60 sec = 0.24 Kg/sec T = 273 + 30 = 303 K

P = [( ) -1 ]

8.41 0.7* 0.24 8.314 303* * 1 1.3 (0.2857) = 7.99 Kwatt ≃ 8 Kwatt Use 2 × 8 = 16 Kwatt

Primary Sedimentation tank

Designing of Primary Sedimentation Tank

DWF = Qave = N × q = 1250000*280 = 350000 m3/day Use 4 sedimentation tanks ⇒ Q = 350000 / 4 = 87500 m3/day Assume surface overflow rate SOR = 40 m/day SOR = Q / A ⇒ A = Q / SOR = 87500 / 40 = 2187.5 m

Area = *D ⇒ 2187.5 = *D ⇒ D = 52.8 m 53 m

4 Π 2 4 Π 2 ≃

Area = *(53) = 2206.2 m

4 Π 2 Volume = Q × t = 8750024 *2 hrs. = 7291.67 m Depth H = Volume / Area = 7291.67 / 2206.2 = 3.30 m

Total depth = 3.30 + 0.5 = 3.80 Use H = 4 m Check SOR ⇒ SOR = Q /A = 87500 / 2206.2 = 39.66 mg/day OK (20-80)mg/day

Mass Balance

From Figures 23-1 and 23- We can find SS and BOD removals % SOR = 39. ∴ BOD removal = 36% and SS removal 57% For BOD Removal ∗ BOD input = 380 × 87500 = 0.38 * 87500 = 33250 Kg/day ∗ BOD Removal = 0.36 × 33250 = 11970 Kg/day ∗ BOD output = 0.64 × 33250 = 21280 Kg/day For SS Removal ∗ SS input = 0.4 × 87500 = 35,000 Kg/day ∗ SS Removal = 0.57 ×35,000 = 19950 Kg/day ∗ SS output = 0.43 × 35,000 = 15050 Kg/day ∗ Assume percentage of Solids in Sludge = 5% ∗ Assume density of Sludge = 1017 Kg/day ∗ Mass of Solids = SS in sludge / SS % = 19950 / 0.05 = 399, ∗ Q sludge = Mass of Sludge / ρ sludge = 399,000 / 1017 = 392.33 m3/day Concentration of BOD ∗ BOD input = 0.38 kg/m ∗ BOD Removal = 11970 / 392.33 = 30.51 Kg/m ∗ BOD output = = 0.244 Kg/day = 244 Kg/L 21280 87500−392. Concentration of SS

∗ SS input = 0.4 Kg/m ∗ SS Removal = 19950 / 392.33= 50.84 Kg/m ∗ SS output = = 0.172 Kg/day =172 Kg/L 15050 87500−392. References :Delft University of Technology Faculty of Civil Engineering and Geosciences Department of Water Management Section of Sanitary Engineering Stevinweg 1 2628 CN Delft www.sanitaryengineering.tudelft.nl