UNIT II
ESTIMATE OF OTHER STRUCTURES
Estimating of septic tank, soak pit – sanitary and water supply installations – water supply pipe line – sewer line – tube well – open well – estimate of bituminous and cement concrete roads – estimate of retaining walls – culverts – estimating of irrigation works – aqueduct, syphon, fall.
2.1 Estimating of septic tank, soak pit
Septic Tank:
In rural areas where houses are spaced so far apart that a sewer system would be too expensive to install, people install their own, private sewage treatment plants. These are called septic tanks. A septic tank is a key component of the septic system or sewer system.
Septic tanks are generally installed by isolated communities like schools, hospitals & institutions. Septic tank may be defined as primary sedimentation tank. Having capacity to remove 60 to 70% dissolved matter.
Section of a typical septic tank
Construction Details:
Septic Tank Collection System
Tank is designed to prevent direct currents between the tank inlet and outlet. T-pipes or baffles at inlet & outlet with submerged ends to stop current and retaining the scum. Manhole covers are provided in top slab to permit the inspection & maintenance. Top of the tank is covered with RCC slab. Sometimes stairs steps are also provided. Provide vent pipe to escape foul gases. Side sump-well with portable pump.
Design Considerations:
Capacity of septic tank:
Capable of storing sewage flow during detention period &extra volume of sludge for 0.5 to 3 yrs.
Inlet and outlet tees:
Detention period:
12 to 36 hrs (adopted 24hrs).
Length to width ratio:
Length is 2 to3 times of width.
Working:
Sewage enter through the Inlet. There are three layers are:
1. Anything that floats rises to the top and forms a layer known as the scum layer.
2. Anything heavier than water sinks to form the sludge layer.
3. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.
Organic matter is digested by bacteria. Digestion of settled sludge is carried out by Anarobic decomposition also called sludge digestion. Due to decomposition foul gases are produced thus mass of sludge is decreased. BOD of water also decreased .
Cleaning:
Regular septic tank cleaning is the only way to prevent septic tank systems from clogging and well. Pumping is only way of cleaning. Frequency of cleaning depends on volume of solids, amount of indigestible solids and the ambient temperature.
Potential Problems:
Oils and grease are difficult to degrade can cause the inlet drains to block. Flushing non-biodegradable items (cigarette butts & cotton buds) clog a septic tank. Chemicals (pesticides, herbicides)and inorganic materials(paints or solvents)damage the working of a septic tank. Roots from trees may clog and rupture them. Excessive water entering the system will overload it and cause it to fail. Septic tanks by themselves are ineffective at removing nitrogen compounds that can potentially cause algal blooms in receiving waters. All toilet paper are not suitable to dispose in septic tank.
Environmental Issues:
Organic materials +Nutrients = CH4 + CO2 +NH3 +Biomass
Sulfates, under the anaerobic conditions of septic tanks, are reduced to hydrogen sulfide (H2S), a pungent and toxic gas. Nitrates and organic nitrogen compounds are reduced to ammonia. Because of the anaerobic conditions, fermentation processes take place, which ultimately generate carbon dioxide and methane. Methane, a potent greenhouse gas. Waste water also contain phosphate and phosphate can be the limiting nutrient for plant growth in many ecosystems.
Advantages:
1. Ease in construction.
2. No maintenance problem(except periodical cleaning)
3. Excellently functioning ST can considerably reduce suspended solids &BOD from sewage.
4. Due to digestion reduction in volume is about 60% &reduction in weight is about 30%.
5. It is suitable for isolated community.
Disadvantages:
1. If ST is not properly functioning, effluent will be very foul &dark.
2. Leakage of gases from top cover of ST may caused bed smells &environmental pollution.
3. Periodical cleaning, removal and disposal of sludge remains a tedious problem.
Disposal Of Effluent:
Ways Of Disposal Of Effluent Are:-
i) Soak Pit
ii) Sub-Surface Irrigation
iii) Leaching Cess-Pools
iv) Treated In Trickling Filters
v) Recycling.
Soak pit:
A Soak Pit is a covered, porous-walled chamber that allows water to slowly soak into the ground. Pre-settled effluent from septic tank is discharged to the underground chamber from where it infiltrates into the surrounding soil.
Design:
A layer of sand and fine gravel is spread across the bottom to help disperse the flow.
Depth should be between 1.5 and 4m deep, but never less than 1.5m above the ground water table.
The Soak Pit is filled with coarse rocks and gravel.
The rocks and gravel will prevent the walls from collapsing, but will still provide adequate space for the wastewater.
A-masonry ring, B-stone or brick aggregate, C-brick chamber, D-30cm thick outer casing with coarse sand, E-effluent from septic tank
Working:
As wastewater percolates through the soil from the soak pit , small particles are filtered out by the soil matrix and organics are digested by micro-organism.
Soak pit are best suited to soils with good absorptive properties. Clay, hard packed or rocky soils are not appropriate.
Adequacy:
A Soak Pit does not provide adequate treatment for raw wastewater and the pit will clog quickly.
A Soak Pit should be used for discharging pre settled black water or grey water .
Soak pits are appropriate for rural and suburban settlements.
They depend on soil with a sufficient absorptive capacity. They are not appropriate for areas that are prone to flooding or have high groundwater tables.
Advantages:
Can be built and repaired with locally available materials.
Small land area required.
Power conservative.
Can be built and maintained with locally available materials.
Simple technique for all users.
Disadvantages:
Pretreatment is required to prevent clogging,although eventual clogging is inevitable.
Negatively affects soil and groundwater properties.
Let us design a Septic tank for a colony with 100Quarters. Assume all other data required.
Given:-
No Of Qrs = 100Nos
Assumed:-
Water supply=200lit/per/day
No of persons per Qrs = 5Nos
Sewage generation = 80% of water supply
Detention period = 18hours
Cleaning period = once in a year
L:B = 4:1 & Depth of Storage of water = 1.8m
Sludge deposit = 30lit/person/year
Min Free Board required = 30cm (Rec= 50cm)
Septic Tank Calculation
Total Waste water coming to septic tank = 100*5*200*80/100=80000 lit/day
Detention period = 18hours
Capacity of tank required =80000/24*18=60000lit
Capacity req for sludge accumulation = 30*5*100 = 15000lit / year
Total capacity req = 60000 + 15000= 75000lit
Plan area of the Septic tank = 75/1.8=41.67m2
L:B taken as 4:1, 4B*B= 41.67,B =√ ( 41.67/4)
B = 3.25m. L = 3.25* 4 = 13m.
Total depth of Septic tank = 1.8 + 0.5 = 2.30m .
Soak Pit Design Calculation
Waste water coming out from septic tank= 80000lit / day
Percolation rate = 1500 lit / m3 / day
Volume of filter media = 80000 / 1500 = 53m3
Depth taken = 2.5m
Area of soak pit = 53/2.5 = 21.2 sqm
Dia of Soak well req= √21.2*4 / π = 5.2m