Engineering design of sewage treatment plant in a city of Hebei Province

I. Overview of the Project

A sewage treatment plant serves about 500,000 people with a catchment area of 40 square kilometers. The design scale is phase I160,000 m3/d and long-term 320,000 m3/d, which is constructed with foreign loans. Among the urban sewage, domestic sewage accounts for 35% and industrial sewage accounts for 65%, which is discharged to the suburbs through pipelines and then discharged into the surrounding rivers through 37 kilometers of open channels.

2. Design water quality and discharge quality

1. Designed to treat water quality and quantity.

The designed processing capacity is 160000m3/d (the maximum processing capacity is 208000m3/d).

Due to the changes of urban drainage system and actual water inflow, its sewage treatment capacity has been kept at around 130000m3/d for several years. In the influent quality, the domestic sewage quality is relatively stable, while the industrial wastewater quality fluctuates greatly. See the table below for the actual inlet and outlet water quality of the sewage plant.

Project BOD5 (mg/L) COD (mg/L) SS (mg/L) pH Toxic substance heavy metal.

Water inflow100-200150-350 80-200 7-9-trace

Effluent ≤30≤ 120≤30 6-9- trace

The design inlet water quality is (excluding toxic substances and heavy metals)

BOD5 200mg/L COD 400mg/L SS 250mg/L pH 7-9.

2. Emission standards

The effluent quality meets the national secondary discharge standard, and the designed effluent quality is

BOD5 ≤ 20mg/l COD ≤120mg/l SS ≤ 25mg/l pH value 6-9.

Iii. Selection and flow of treatment process scheme

1. Principles for determining the treatment process

In order to achieve the goal of efficient and stable operation, low capital investment and low operating cost of sewage treatment plant at the same time, the selection of sewage treatment process scheme follows the following principles:

(1) The technology is mature, the treatment effect is stable, and the effluent quality is guaranteed to meet the discharge standard;

② Low investment, low operating cost, low investment and high benefit;

③ The selected technology and equipment are advanced and reliable, with high degree of localization and good performance.

2. Determination of treatment process

The common activated sludge process was adopted.

After the sewage enters the factory, it enters the collecting basin with automatic coarse grid, and a submersible pump is installed in the collecting basin. After the sewage is lifted, it enters the aerated grit chamber to remove sand grains, and then most of the suspended solids are removed through the primary grit chamber. The initial submerged water enters the aeration tank through the elevated canal in the factory. The aeration tank adopts the form of circulating push-flow reaction, and its effluent is separated by the advection secondary sedimentation tank and discharged into the surrounding rivers.

First, the primary sludge and secondary surplus sludge enter the pre-concentration tank, and then enter the egg-shaped digestion tank for medium-temperature digestion to stabilize the sludge. The digested sludge is further concentrated in the post-concentration tank to reduce the volume, dehydrated by belt filter press, and the mud cake is transported abroad for disposal.

3. Brief introduction of treatment process

Activated sludge process is an aerobic treatment process. Sewage is aerated and oxygenated in the aeration tank, so that various activated sludge microorganisms grow and multiply in large quantities, bacteria that can form bacterial micelles form flocs, protozoa attach to them, and filamentous bacteria and fungi interweave with each other to form floc particles suspended in the mixed solution, and each particle is a microbial population. This kind of activated sludge particles contact with the sewage entering the aeration tank, that is, absorb, decompose and adsorb the pollutants in the sewage. After a period of aeration, most of the organic matter in the sewage is assimilated by microorganisms and then enters the sedimentation tank. Flocculated activated sludge particles can settle well to the bottom of the tank, and the supernatant is treated water, which can be discharged from the system. Part of the precipitated sludge is replenished and refluxed to the aeration tank, mixed with untreated sewage, and the above actions are repeated; The other part of sludge is discharged as surplus sludge.

Three. Design process requirements

The process adopts ordinary activated sludge process (or multi-point water inflow).

There is a main gate before the sewage enters the factory, and an overflow pipe is directly discharged before the main gate.

Sewage enters the water collection tank through the automatic coarse grid after entering the factory,

A submersible pump is arranged in the water collecting tank,

After the sewage is lifted, it enters a cyclone grit chamber with a fine grid to remove sand.

Removing most suspended solids through the primary sedimentation tank,

The first-class sewage enters the aeration tank through the elevated channel in the factory. The aeration tank adopt that form of circulating plug flow reaction,

Its effluent is separated by advection secondary sedimentation tank and discharged into surrounding rivers.

The primary sludge and the secondary residual sludge firstly enter a pre-concentration tank,

After concentration, it enters an egg-shaped digester for medium-temperature digestion to stabilize the sludge.

The digested sludge is further concentrated in the post-concentration tank to reduce the volume, dehydrated by belt filter press, and the mud cake is transported abroad for disposal.

Fourth, the engineering design

1. General layout design

(1) plane layout principle

The general layout includes sewage and sludge treatment, process structures and facilities, various pipelines, pipelines and channels, and various auxiliary buildings and facilities. The general layout shall follow the following principles.

1. The layout of treatment structures and facilities shall conform to the technological requirements, and be centralized and compact to save land and operation management.

2. The facilities of process structures and auxiliary buildings without changing different functions should be arranged independently according to different functions, and the relationship with environmental conditions (such as terrain trend, sewage outlet direction and wind direction) should be coordinated.

3 building spacing should meet the requirements of traffic, pipeline (canal) laying, construction and operation management.

4. The plane layout of pipelines (lines) and passages should be coordinated with their vertical layout, meet the requirements of various media transportation in sewage treatment plants, and try to avoid repeated hoisting and twists and turns, so as to facilitate energy saving and consumption reduction and operation and maintenance.

5. Coordinate the relationship between auxiliary buildings, roads, greening and treatment structures, so as to facilitate production and operation, ensure safe and smooth operation and beautify the factory environment.

(2) the characteristics of plane layout

1. Compact layout and clear streamline.

2. The living area, sewage area and sludge area are clearly divided, and the comprehensive building is built from the gate to form the living area at the entrance. The area is located in the upwind direction of the dominant wind direction, far away from the grille and sludge area, with enhanced greening and good environment.

3. The sludge area is located in the downwind and the lowest corner of the plant. The digester is far away from the building and does not affect other facilities.

4. The auxiliary production area is close to buildings that need maintenance and electricity, which is convenient for workers.

5. The road design of the factory takes into account that employees can reach anywhere smoothly.

6. There is a back door, and the grid slag, gravel and mud cake produced in the production process are transported away through the back door instead of the front door, thus avoiding the influence of the cleanliness of the living area at the gate.

The technological process of sewage treatment is composed of several unit treatment structures (equipment) with different functions, water pipelines and canals. With the development of sewage treatment technology, on the one hand, the types of treatment facilities with the same function are increasing, on the other hand, the treatment functions of the same facilities are also expanding. After the process flow and structural type of sewage treatment plant are determined, the process calculation task of sewage treatment is mainly to determine the geometric size and quantity of structures (equipment) and pipelines, as well as the specifications and dosage of auxiliary equipment, materials and chemicals. So as to provide a basis for the layout of the treatment plant.

① The design scale of Qingdao Licunhe Sewage Treatment Plant is 17× 104m3/d, and the bottom of the grille is 8.0m above the ground. The coarse grid room is semi-underground, and there are three mechanical coarse grids, with a grid spacing of 25mm and a grid width of1.36m.. Grille slag intercepted by the grille is collected by the belt conveyor, lifted by the screw conveyor, and then enters the ground grille slag box. A maintenance platform with a width of 1.0 m is set near the water surface of the grille. The four fans are located in the semi-underground room, the air inlets are located in the passages and rooms, and the air volume of the fans is 8000m3/h. The sewage flowing through the coarse grille is lifted by the lifting pump room and then enters the fine grille room. Three stepped mechanical grates are designed between the fine grates, with a spacing of 6 mm and a width of1.28 m. The fine grates are dewatered by a screw compactor and then transported outside. ② The design scale of Xinxin Board Sewage Treatment Plant in Hohhot is 10× 104m3/d, and the bottom of the grille is 5.4m above the ground. The coarse grille room is floor-standing, with two mechanical grilles. The gap between the grilles is 25mm, the design width is 2.0m, and the height is 8.4m The roof is provided with a 2.5m× 1.5m skylight, which reduces the height between the grilles from 1 1.5m to 6.2m The air inlet of the exhaust fan is located where maintenance personnel often appear in the water passage. * * * There are two exhaust fans with a ventilation capacity of 8 250m3/h. ..

Process flow:

Third, the main structure

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Serial number?

Name?

Specifications (meters)?

Number (seat)?

design parameter

Zazie Hoko

Main equipment

1 grille l× b = 3.16×1.65 2m flow q = q =165600m3/d/d.

Grille clearance b= 15mm, grille crossing speed v= 1.0m/s, two mechanical slag removers.

2.L×B×H= 10×8×5 1 meter flow q = q =165600 m3/d/d.

Single pump flow Q=2400m3/h submersible sewage pump 4 manual hoists.

3 grit chamber l× b =18× 3.222m flow q = q =165600m3/d/d.

Horizontal velocity v=0.3m/s effective water depth h= 1.0m sand-water separator.

4 Primary sedimentation tank L×B=×27×6 2 Flow Q= 165600m3/d

Q = 2.0m3/(m2 h) Retention time t= 1.5h Sludge scraper bucket.

5 ?

The aeration tank L×BH=7 1.5×7.55 2 flow Q= 120000m3/d BOD=200, and the removal efficiency is 90%.

6 secondary sedimentation tank d× h = 46./kloc-0 /× 6.152m flow q =120000m3/d.

q = 1.5m 3/(m2·h)

The retention time of the water outlet weir plate of the mud scraper is t = 2.5h.

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(1) coarse grille (two groups, one for use and one for standby)

Function: to remove large floating impurities in sewage and ensure the normal operation of sewage lifting pump, mechanical grille is adopted. Under normal circumstances, two channels run at the same time, and under unexpected circumstances, one channel runs.

Main parameters: design maximum flow qmax = 208,000m3/d = 2.4m3/s.

Gap width of steel bar b = 25.0mm

Water depth before grid h =1.0m.

Grid crossing speed v = 0.8m/s

Grate inclination α = 60.

Grating width S=0.0 1m (grating section is rectangular with sharp edges).

Number of grid gaps:

n== 1 12

Grid slot width:

B=S(n- 1)+bn=3.9 1m

Length of widened part of inlet channel:

Let the inlet width B 1=2.3m, and the expansion angle α1= 20.

l 1 =(B–B 1)/2tgα 1 = 2.2 1m

Length of narrowed part: L2 = l1/2 =1.10m.

Head loss across the grid:

h 1=4/3 ()k=0.06 1m

Total height behind the grid: set the protection height of the channel before the grid h2 =0.3m

h = h+h 1+H2 = 1.36m≈ 1.4m

Total length of grid slot:

l = l 1+L2+0.5+ 1.0+h 1/TGα= 5.56m

Daily grid slag quantity:

When the grid spacing is 25mm, the amount of grid slag is 0.03m3/ 103m3 sewage, and Kz is 1.2.

w = 86400 qmax w 1/ 1000 khz = 5.2 m3/d & gt； 0.2 m3/day

Therefore, it is necessary to use mechanical slag removal.

(2) Collecting basin and lifting pump house

Use rectangular self-filling dry pump house. The water collecting tank and the machine room are separated by a partition wall. Only when the suction pipe and impeller are immersed in water, the machine room can always be dry, which is beneficial to the overhaul and maintenance of the water pump and can avoid the corrosion of bearings, pipe fittings and instruments by sewage.

Design flow qmax = 208000m3/d = 2.4m3/s.

Submersible pump with flow rate of 0.6 m3/s is adopted, with 4 users 1 standby.

The water collecting tank is divided into two compartments, and the total effective volume is the water output of a pump for 8 minutes:

V=qt=288 m3

The effective water depth of the sump is 2.0m

Catchment area F= 144m2, width 10m, length 14.4m, and take 15m.

The required lift of water pump: h = 3.3+0.1+0.2+0.6+0.2+0.6+0.5+0.4+1.5 = 7.4m.

(3) Fine mesh

Function: Remove relatively small floating impurities in sewage and ensure the normal operation of subsequent treatment process.

Build two groups, with design flow q = qmax/3 = 0.8m3/s.

Grid gap e=6mm

Water depth in front of the grid h = 0.8m

Grid crossing speed v= 1.0m/s

Grate inclination α = 60.

Calculate with coarse grid:

The number of grid gaps n= 155.

Door slot width b = 2.47

The length of the widened part of the inlet is l1=1.33m.

Length L2 of narrowed part = 0.66m.

Head loss h1= 0.633m.

The total height behind the grid is h =1.73m..

The total length of grid groove L=4. 12m.

Sunrise slag quantity w = 5.2m3/d >; 0.2 m3/day

Therefore, mechanical slag removal is needed.

(4) Cyclone grit chamber

Function: Sewage enters from the tangential direction of the grit chamber at a certain speed, and the sand particles generate centrifugal force, and the sand particles with higher density settle to the sand collection hopper at the bottom of the chamber along the unique structure of the wall and grit chamber. The washing system can prevent the sand in the sand collection hopper from hardening, separate the organic particles attached to the sand particles from the sand particles, and make the organic particles return to the sewage from the sand collection hopper. The rotation of the blades makes the water flow in a complicated vortex state, resulting in a slight rising speed, which drives the organic particles to flow into the next process with the water flow for treatment. By changing the rotating speed of the blades and the gap between the sand collection hoppers, the sand settling effect and the separation effect of organic particles in the grit chamber can be optimized. The advanced air lifting system (or mortar pump) lifts the settled sand in the sand collection hopper to the shaftless spiral sand-water separator, thus completely separating the sand particles from the sewage.

In operation, the cyclone grit chamber system has the advantages of fast inflow and outflow speed, large treatment capacity, good sand removal effect, small floor space, simple equipment structure, energy saving, reliable operation, PLC control, centralized control, continuous automatic operation and convenient operation and maintenance of the whole system, which is suitable for large, medium and small sewage treatment plants and a good substitute for horizontal grit chambers in domestic sewage treatment.

Main parameters: design flow qmax = 20.8 208,000m3/d = 2.4m3/s.

Design residence time t=60s

The velocity of the inlet pipe v1= 0.3m/s.

The rising speed of water flow in the pool v2 = 0.06/s.

Height of cone bottom of grit chamber H4 =1.5m.

Superelevation h1= 0.5m.

Distance from bottom of central pipe to grit surface = 0.3m

It should be divided into three ponds with water inflow and sediment deposition, n=3.

① Diameter of water inlet pipe:

D = = =1.84m.

② grit chamber diameter:

D = = = 4.52m

Height of water flow part:

h2= v2t = 0.0660 = 3.6m

Required volume of precipitation part:

V== 10.37 m3

(5) the actual volume of frustum of a cone:

V 1=

⑥ Total height of pool:

h = h 1+H2+H3+H4 = 0.5+3.6+0.3+ 1.5 = 5.9m

(5) Primary sedimentation tank (radial flow)

The radial sedimentation tank is circular and adopts the form of central water inlet and peripheral water outlet. Water flows horizontally around the pool, the mud bucket is located in the center of the pool, and the bottom of the pool is inclined to the center. Sludge is usually removed by scraping (or sucking) machinery. Radial sedimentation tank adopts mechanical sludge discharge, which runs well and has simple equipment. The sludge discharging equipment has the advantages of molding products.

Main parameters: design flow qmax = 20.8 208,000m3/d = 2.4m3/s.

Surface load q=2.0m3/(m2h)

Number of pools n=3

Precipitation time t=2h.

Precipitated water area:

F=Qmax/nq= 1440m2

Pool diameter:

D = = 42.8m

Effective water depth of sedimentation part:

h2=qt=4m

Effective volume of precipitation part:

V==6480m3

Required volume of sludge part:

V=SNT/ 1000n=20.83m3

Sludge bucket volume:

Let the upper radius of the sludge bucket r 1=2m, the lower radius of the sludge bucket r2= 1m, and the inclination angle =,

h5=(r 1-r2)tg= 1.73m

Sludge bucket volume: v1= H5 (r12+r1R2) =12.7m3.

⑦ Amount of sludge in the cone above the sludge hopper:

If the bottom radial slope is 0.05, the cone height

H4 = (r-1) 0.05 = 0.97m.

Cone sludge volume:

V2 = H4(R2+RR 1+r 12)= 504.8 m3

⑧ Total volume of sludge bucket:

V = v1+v2 = 517.5m3 > 20.83m3.

Pet-name ruby sedimentation tank total height:

Let h 1=0.3m and h3=0.5m

H=h 1+h2+h3+h4+h5=7.5m

Edge height of sedimentation tank:

h′= h 1+H2+H3 = 4.8m

Attending the diameter-depth ratio:

= 10.7 meets the requirements

(6) Aeration tank

The aeration tank is in the shape of oxidation ditch and divided into two groups, each group is arranged into four corridors, and each corridor is 82 ~ 88? M, width 9.5? M, the water depth is 7? M, the volume of each group is 22? 284? M3, total volume 44? 568 cubic meters. The average hydraulic retention time is 5.1h. In the aeration tank, sewage is forced to form a circulating flow, and its flow pattern has the dual characteristics of plug flow and complete mixing. Therefore, it not only has strong impact resistance, but also is not easy to short-flow. ? The aeration and oxygenation system adopts jet aerators with 638 nozzles, which are divided into 8 groups and arranged at the bottom of each corridor. 1 group. Each group is provided with working medium by 1 pump, of which 6 pumps use returned sludge and 2 pumps use mixed liquid of aeration tank. The aeration system belongs to mesopore and micropore aeration, and the air sent by the blower is fully mixed with activated sludge in the ejector and then diffused to the pool surface, so it has high oxygen utilization rate. Under standard working conditions, the dynamic efficiency of aeration system can reach 2.2? kg？ O2/(kW？ H) The working medium of the ejector promotes the water circulation in the pool and keeps the sludge suspended.

Main parameters: design flow qmax = 20.8104m3/d = 2.4m3/s.

Intake water quality: BOD5 BOD5 200mg/l COD 400mg/l SS 250mg/l/l.

Effluent quality: BOD5 ≤ 20mg/l COD ≤120mg/l SS ≤ 25mg/l.

Sludge reflux ratio: R=0.5

① Treatment efficiency:

E=La-Lt/La* 100%=90%

② Aeration tank volume:

If the concentration of suspended solids in the mixed liquor is 3g/L and the coefficient f=0.7, Nw=0.73=2. 1kg/m3, and the sludge load Fw=0.4.

Aeration tank volume V=QLr/NwFw=44568m3.

③ Nominal residence time:

Tm=V/Q=0.2 14d=5. 1h

Ts=V/( 1+R)Q=3.4h

④ Sludge output:

Let the sludge proliferation coefficient a=0.6 and sludge oxidation rate b=0.08.

Y=aFw-bVNw= 14977kg/d

⑤ Mud age:

Tw= 1/(aFw-b)=6.25d

6. Oxygen demand of aeration tank:

Let a 1=0.5kg of BOD and b1= 0.16 kg/kg mlss * d of sludge itself.

o = a 1 qlr+b 1 vnw = 33695kg/d

(7) Secondary sedimentation tank

The advection sedimentation tank has good sedimentation effect, simple construction and low cost.

Main parameters: design water quantity: qmax = 20.8104m3/d = 2.4m3/s.

Surface load: q= 1.5(m3/m2h)

Hydraulic retention time: t=2h

Sludge concentration: x = 3500mg mg/L.

Sludge reflux concentration: x1=10000mg/L.

Number of pools n=4

(1) the effective area of precipitation part:

A=Qmax/nq= 1445m2

② Effective water depth of precipitation part:

h2=qt=3m

③ Effective volume of precipitation part:

V==4333m3

④ swimming pool length:

Let the horizontal speed be 0.004 m/s.

L = vt * 3.6 = 28.8m。

⑤ Pool width:

B = A/L = 50.2

⑥ Total sludge demand:

Let T=2 days, and take S=0.5 liter/person * day per person per day.

V = SNT/1000 = 500m3.

⑦ Sludge bucket volume:

Let the upper radius of the sludge bucket r 1=2m, the lower radius of the sludge bucket r2= 1m, and the inclination angle =,

hs=(r 1-r2)tg= 1.73m

Sludge bucket volume: v1= hs (r65438+02+r65438+0r2+R22) = 43.5m3.

⑧ Amount of sludge in the cone above the sludge hopper:

If the bottom radial slope is 0.05, the cone height

H4 = (r-1) 0.05 = 0.97m.

Cone sludge volume:

V2 = H4(R2+RR 1+r 12)= 527.6 m3

Pet-name ruby sludge bucket total volume:

v = v 1+V2 = 57 1. 1 m3 & gt； 500 cubic meters

Attending the total height of sedimentation tank:

Let the height of the buffer layer H3 = 0.5m

H=h 1+h2+h3+h4+h5=6.5m

Edge height of sedimentation tank

H'=h 1+h2+h3=3.8m

(8) Sludge concentration tank

Adopt continuous flow gravity concentration tank, which is circular and vertical.

Main parameters:

The total sludge output is14977 kg/d/d.

Water content ρ=99.2%, concentration =40Kg/m3.

After shrinkage: sludge concentration 40g/L, moisture content ρ=96%.

The effective water depth of the concentration tank is h=4m.

Concentration time 10h

(1) Concentration of sludge after mixing:

c =( 127368.5+224 140)/ 14977 = 13.2kg/m3

② Concentration pool area:

Let the solid flux be M = 55Kg/m2d.

A==847m2

③ Diameter of concentration tank:

D = =19.5m.

④ Height of working part of concentration tank:

h 1==3.7m

⑤ Total height of concentration tank:

Set the superelevation h2=0.3m, the buffer height h3=0.3m, and the height of the concentration tank.

H = h1+H2+H3 = 3.7+0.3+0.3 = 4.3m.

(9) Digester

The sludge digesters are egg-shaped and have the same volume. There are ***3 sludge digesters, each with a maximum diameter of 24? M, the total height is 42.93? M, liquid height 40.93? M, each volume 10400? Cubic meters. The digester adopts medium temperature digestion, and the sludge heating system consists of two biogas boilers, three heat exchangers and three sludge circulating pumps. The maximum designed biogas production is 13000? Cubic meters/day.

Compared with other digesters, the egg-shaped digester has the following characteristics: ① The bottom of the digester is not easy to accumulate sand or mud, so the effective tank capacity will not be reduced; (2) it is easy to stir and mix, and there is no dead zone in the tank, which can maximize the effective tank capacity; For the same mixing effect, the energy consumption of mixing and stirring is lower than other tank types; (3) the upper part is not easy to accumulate slag; (4) For the same volume, its surface area is smaller than other pool types, so the heat loss is small; ⑤ The structure is stable and cracks are not easy to appear; ⑥ The swimming pool is streamlined and beautiful.

(10) Sludge concentration filter press room

Function: Concentrate, filter and dehydrate the surplus sludge, so as to reduce the water content of sludge as much as possible, reduce the volume of sludge and facilitate loading and unloading operations. Use a belt filter press.

Belt filter press is an efficient solid-liquid separation equipment based on chemical flocculation, contact filtration and mechanical extrusion. Because of its advantages of simple process flow, high degree of automation, continuous operation, simple control operation and adjustable working process, it is being widely used. The flocculated sludge first enters the gravity dewatering area, and most of the free water is filtered by the filter belt under the action of gravity; With the operation of the filter belt, the sludge enters the wedge-shaped area composed of two filter belts, and the two filter belts slowly pressurize the sludge, so that the sludge gradually thickens, the fluidity decreases, and the sludge transitions to the pressurized area; In the extrusion zone, the sludge is subjected to the increasing extrusion force and the shear force generated by the alternate change of the upper and lower positions of the two filter belts, and most of the remaining free water and interstitial water in the sludge are filtered out, and the sludge becomes a flaky filter cake with low water content; The upper and lower filter belts are separated by the discharge roller. With the change of the curvature of the filter belt, the filter cake is scraped off by the scraper to realize the solid-liquid separation of materials. The upper and lower filter belts are cleaned and reused for the next round of concentration and pressure filtration.

There are 1 building with a plane size of 66m×40m. The daily dry weight of discharged sludge is 18600kg/d, the flow rate of residual sludge mixed liquor is 2360m3/d, the water content of sludge inlet is 92%, and the water content of sludge outlet is 78%. The main equipment is 8 belt-type concentration filter presses with a bandwidth of 2.0m, with a single processing capacity of 25 m3/h in the concentration section and 9 m3/h in the filter press section, and the designed working time is10 h.