CE3205 Lecture 11 Sewerage System Design (1)

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Sewerage system design

Calculation waste water flow rate based on population.Calculate BOD5 to indicates strength and weakness of wastewater.Design criteria of sewer system based on UK standard.Waste water treatment processFoul Sewerage(Sanitary)The main flow in a foul sewerage system is generally the polluted water-borne waste from domestic buildings,Consider the connection from the house to the sewer, which is known as a house connection or drain.

DrainsTo determine wastewater flows in building drainage.Following the guidance in BS EN 752 Drain and sewer systems outside buildings, and BS EN 12056-2 Gravity drainage systems inside buildings - Part 2: Sanitary pipework, layout and calculation.

Types of sewerFor intermittent use a factor of 0.5 may be applied, so in a total of N dwellings, the flow is then given byQ [litres/second]=0.5 (3N)subject to a minimum design flow of 1.6 litres/second, and with the depth of flow d 0.7 D to allow ventilation.

Self cleansing conditions are usually ensured by simple gradient rules, such as a 100 mm pipe at a slope of 1 in 40.

Building Regulations Approved Document H (reprinted 2010, free to download from http://www.planningportal.gov.uk), which follows a similar approach to that described above.

Manufacturers organisations also publish useful information, such as the Clay Pipe Development Association (CPDA) booklet entitled The specification, design and construction of drainage and sewerage systems using vitrified clay pipes.

REFERENCESFoul SewersFor sewerage systems, wastewater flows are usually determined on a population basis, as described for the UK in the following sections.Traditionally in UK, the average foul flow in a sewer is known as the dry weather flow , DWF, and may be expressed

DWF = P.G + I + E

*The average daily domestic flow per person (G) in UK is given as ranging from 150 to 300 litres/day (BS EN 752:2008 p.97, Table E.4).

P = populationG = wastewater flow per personI = infiltrationE = industrial effluent/sewage

Criteria for Population Equivalent (PE) Calculation (MS-1228)Foul sewer design flows by the population methodFoul sewers in UK are designed for up to 6 times of DWF (BS EN 752:2008, p.98, Table E.5). This is dependent on catchment size, and additional allowance is made for infiltration.For new housing areas in UK, the following is usually assumed:3 people per dwelling200 litres per person per day ( G )10 % infiltration ( i.e. I = 10 % of P.G, so multiply P.G by 110/100 )6 times peaking factorSo the peak flow per dwelling :-= 6xDWF =6( P.G + I + E)= 6 [3 x 200 x 110/100] 4000 litres/day = 0.046 l/sThis is referred to as the design flow.A lower peak of two times the average (2 DWF) is assumed to occur at least once daily (see BS EN 752:2008 p.150, NA.11.2.3.1), and is used for minimum velocity considerations, as described in the following section.

Self cleansing velocities for foul sewers

BS EN 752 (2008) recommends a velocity of at least 0.7 m/s for pipes up to 300 mm diameter (p.61, 9.6.3.1), but higher velocities may be needed in larger pipes (p.150, NA.11.2.3.1). A flow of two times the average domestic flow is used for this calculation.Sewers for Adoption (SfA) requires a velocity of 0.75 m/s at one-third of the design flow. (One third of 6DWF equals 2 DWF, which is the flow at which velocity should be checked). Where this velocity requirement cannot be met, then the self cleansing criterion would be considered to be satisfied if one of the following gradient rules applies:a 150 mm nominal internal diameter gravity sewer having a gradient not flatter than 1 in 150 where there are at least ten dwelling units connected.a sewer or lateral drain with a nominal internal diameter of 100mm, or a lateral drain serving ten or less properties, laid to a gradient not flatter than 1:80 where there is at least one WC connected, or 1:40 if there is no WC connected.

Pipe roughnessThe typical roughness value used for foul gravity sewer design is ks = 1.5 mm, for any pipe material. In time, pipes slime up and give a similar roughness, regardless of their material.

From experimental work on clay and pvc pipes reproduced in CPDA (2001)Other practical design criteria for foul sewerageThe minimum pipe diameter for foul sewers in the UK was traditionally 150 mm, but now 100 mm is allowed for ten properties or less. 100 mm is the traditional size for individual house connections. Manholes are required at changes in size, slope or direction; The maximum spacing is 100m, although this may be increased for man entry sizes (greater than 900 mm diameter), but is often less than this to accommodate branches and bends.At manholes where pipe sizes change it is usual to align the levels of the pipe soffits(highest point of internal surface).

Slopes to be considered in design are: the natural ground slope,the required self cleansing gradient,the slope to achieve or maintain minimum cover.

A useful approximate rule is that a D (mm) diameter sewer laid at a slope of 1 in D will give a velocity approximately equal to the self cleansing value. This gradient rule is mentioned in BS EN 752:2008 p.61, 9.6.3.1.

e.g. 225 mm sewer with roughness k = 1.5 mm laid at 1 in 225 gives a pipe full flow of 30 litres/s and pipe full velocity of 0.75 m/s.

A pumping station may be needed to lift foul flows from a new development into the existing sewer system, but is best avoided if possible.

Waste water treatment

Processes that combine physical, chemical and biological methods.

Wastewater treatment plants are usually classified as providing primary, secondary or tertiary (or advanced) treatment, depending on the purification level to which they treat.Waste water treatment

*BOD5 = amountoforganic pollution(that can be oxidized biologically) in asampleof water.

CHEMICAL CHARACTERISTIC

Alkalinity: an indicator of wastewaters capacity to neutralize acids, alkalinity is measured in terms of bicarbonate, carbonate and hydroxide alkalinity. Alkalinity is essential to hold the neutral pH of the wastewater during biological treatment.

Biochemical oxygen demand (BOD): an indicator of the amount of biodegradable matter in the wastewater, normally BOD ismeasured in a 5-day test conducted at 20C (BOD5) and normally ranges from 100 to 300mg/L

Chemical oxygen demand (COD): an indication of the amount of oxidizable matter present in the sample, the COD is normally in the range of 200 to 500 mg/L. Industrial wastes present in the wastewater can significantly increase this.

Dissolved gasses: the specific gases and normal concentration dissolved in wastewater are based on wastewater composition, and under septic conditions may typically include oxygen in relatively low concentration, carbon dioxide, and hydrogen sulfide.

Nitrogen compounds: nitrogens type and amount vary from raw wastewater to treated effluent, but nitrogen oxygen is mostly found in untreated wastewater in the forms of organic nitrogen and ammonia nitrogen (presence and levels determined by laboratory testing).

Total Kjeldahl nitrogen (TKN): the sum of these forms of nitrogen. Normal wastewater contains 8 to 35 mg/L and ammonia nitrogen ranging from 12 to 50 mg/L.

pH: pH express wastewaters acid condition. For proper treatment, wastewater pH should generally range from 6.5 to 9.0.

Phosphorus: in secondary treatment process, phosphorus must be present in at least minimum quantities or the process wont process. However, excessive phosphorus causes stream damage and excessive algal growth. Phosphorus normally range 6 to 20 mg/L.

Solids: most wastewater pollutants can be classified as solids, and wastewater treatment is generally designed to either remove solids, or convert them to more stable or removable forms. General practice classifies solids as setteleable, floatable, or collodial. Total solids concentration in wastewater normally ranges from 350 to 1,200 mg/L.

Remove 90-95% settleable solids 40-60% total suspended solids, and 25 to 35% BOD

Removes gravel, sand, silt and other gritty materials

Reduces odors, neutralizes acids, reduces corrosion,reduces BOD5, improves solids and grease removal, reduces loading on the plant and aids subsequent processes.Produces effluent/sewage with not more than 20 mg/L BOD5 and 30 mg/L suspended solidsRemoves pollutants, including nitrogen, phosphorus, soluble COD and heavy metals to meet discharges or reuse criteria with respect to specific parametersDiagram of sewage treatment process cycle

Example of calculating average daily wastewater flow rateQuestion: Table shows the information of daily wastewater flow rate and BOD5 concentration for an urban area. Determine the average daily wastewater flow rate and BOD5 concentration.UtilityPopulation Equivalent (PE)UnitAverage Flowrate (l/day)BOD5 (kg/day)

Domestic30 000per capital6250.1Hospital650 bedper capital6000.15Restaurant250 customers per day per capital300.03School with cafe1500 students each per student2800.1School without cafe5400 student each per student600.03UtilityPE x average daily flowrateDomestic30 000 x 62518,750,000Hospital650 x 600390,000Restaurant250 x 307500School with cafe1500 x 280420,000School without cafe5400 x 60324,000Total wastewater flow19,891,500 (litre/day)Solutioni. Calculate the average wastewater flow rateii. Calculate the average BOD5 concentrationUtilityPopulation Equivalent (PE)BOD5 (kg/day)

PE x BOD5 Domestic30 0000.130000x0.13000Hospital650 bed0.15650x0.1597.5Restaurant250 customers per day 0.03250x0.037.5School with cafe1500 students each 0.11500x0.1150School without cafe5400 student each 0.035400x0.03162Total BOD5 3417kg/dayAverage BOD5 concentration=Total BOD5 /Total wastewater flow rate

341719,891,500=0.000172 Convert unit to mg/LKg/dayLiter/day