Module 1 Every community produces both liquid and solids waste and air emissions. The liquid waste – waste water is consensually the water supply of the community after it has been used in a variety of applications. From the stand point of generation, waste water has been defined as a combination of liquid or water carried waste removed from residences, institution and commercial and industrial establishments, together with such ground water, surface water and storm water as may be present. Sanitary Engineering Sanitary engineering is the branch of civil engineering associated with water, disposal of sewage and other public health services. It consists of scientific and methodical collection, conveyance, treatment and disposal of the waste matter, so that the public health can be protected from offensive and injurious substances. • Waste water engineering is that branch of environmental engineering in which the basic principles of science and engineerijg are applied to solving the issues assossiated with the treatment and reuse of waste water. Sewage, sewer and sewerage system Sewage, is the wastewater resulting from a acomminity. sewage are mainly of 2 types:• Domestic sewage : This consists of liquid wastes originating from urinals, toilets, bathrooms, kitchen sink wash basins etc. of the residential, commercial or institutional buildings. • Industrial sewage : this consists of liquid wastes originating from the industrial process of various industries. The quality of the industrial sewage depends largely on the type of industrial sewage depends largely on the type of industry and the chemicals used. • Storm water : Storm water or storm sewage or storm drainage is the run off resulting from the rainstorm. • Sewer: Sewer is the underground conduit or the pipes through which sewage is carried to a point of discharge or disposal. ‘Drains’ carry stormwater. • Sewerage system: A sewerage system consists of a network of sewer pipes laid inorder to carry the sewage from individual homes to sewage treatment plant. This network of sewers may consist of house sewers(or individual home connections); lateral sewers(or submains); main sewers (generally called trunked sewers);outfall sewer(the sewer which transports sewage to the point of tre atment) Manholes are provided in every sewer pipes at suitable intervals so as to facilitate their cleaning and inspection. In sewers which carry storm waters solely or in combination with sewage, inlet called catchment basins are provided to permit entrance of storm water.
Types of sewerage system: Combined system: when the storm water is taken along with the sewage, it is called combined system. Separate system: when the storm water and sewage are taken independently of each other through two different sets of sewers, it is called separate system. Partially separate system: sometimes a part of storm waters especially that originating from roofs or paved courtyards of buildings, is allowed to be admitted into the sewers. Similarly, sometimes domestic sewage coming out from the residences are allowed to be admitted into the drains, the resulting system is called a partially separate system Combined System Advantages 1) 2) 3) 4) System requires only one set of sewer hence maintainance cost is reduced. Sewer are of large size, hence less chance of chocking. Strength of sewage is reduced by dilution. The foul smell is reduced due to dilution of sewer gas formed as there is more air. Disadvantages 1) 2) 3) 4) 5) 6) Cost of construction is high as large dimension is to be constructed. Handling and transportation of sewer is difficult. Load on treatment plant increases due to inclusion of storm water. Uneconomical when pumping is required to lift sewage. Overflow of sewers during rainy season. Large sewers are more difficult to be ventilated. Separate System Advantages 1) The cost of installation is low. 2) The load on treatment units will be lowered since only sewage is carried to be treated. 3) Economical in cases if sewage is to be lifted. 4) Sewage of more uniform character, hence efficient putrification. Disadvantages 1) 2) 3) 4) 5) The sewers are of small size hence difficult to clean. Likely to be chocked. Two sets of sewers may ultimately prove to be costly. Storm water drains are useful only during rainy season. Foul smell will be more due to small size of sewers. Partially combined sewer
Advantages 1) Sewers are of reasonable size. Their cleaning is therefore not very difficult. 2) Less chances of chocking as storm water is permitted in. 3) The problem of disposing of storm water from homes is simplified. Disadvantages 1) During dry weather velocity of flow is low hence self cleansing velocity is not achieved. 2) Storm water increases the cost of treatment units. QUANITY OF SEWAGE In order to determine the section of the sewer,it is essential to know the total quantity of sewage that would flow through the sewer.The total waste water flow can be divided into two components. (a) Dry weather flow(DWF) (b) Storm water flow. Dry weather flow :The dry weather flow is the flow through the sewers that would be available during non rainfall eriods. It consists domestic sewage, industrial sewage,ground water infiltration wastewater from public facilities. Storm Water flow The additional flow that would occur during rainy season.It consists of runoff available from roofs streets, yards open spaces etc during rainfall. Factors affecting dry weather flow. Dry weather flow, depends on the following factors (a) (b) (c) (d) Rate of water supply. Population growth Type of area reserved. Infiltration of ground water. (a)Rate of water supply: the quantity of waste water produced from a community would depend on the rate of water supply per capita per day. The quantity of waste water entering the sewers will be less than total quantity of water supplied. However private source of water supply(eg:wells) and infiltration of subsoil water in the sewer increase the wastewater flow rate. This extra water that enters the sewers can
be assumed to approximately equal to the water lost. Generally when no extra water enters the sewers, the waste water quantity must be assumed to be 80% of the quantity of the water supplied . the sewers should be designed for a minimum of 150litres/person/day. (b)Population growth: the sewerage system are for the quantity of waste water not only for that present population but also for the population but also for the population of the future .The population can be forecasted by various methods. DESIGN PERIOD: the period for which the components of the sewerage system are designed. Components design period Branches and main sewer 300years Pumps 5-10years (c)Type of area served: the quantity of waste water produced depends upon the weather area to be served in residential,commercial,or industrial. The waste water from the residential area depends upon ,the rate of water supply. The amount of waste water from the industrial locality depends upon the type of industries and their corresponding industries processes. The same in the case with commercial undertakings. (d)infiltration of subsoil water : ground water or subsoil water may infiltrate in to the sewers through the leaky joints. Exfiltration is the reverse process which indicates the flow of waste water from the sewer in to the ground.while due to infiltration the quantity of flow through sewer increases ,exfiltration results in decrease inflow and consequent increase in pollution of ground water. infiltration in more important with respect to sewer design and design of treatment plant. VARIATION IN RATE OF SEWAGE The rate sewage flow is not constant. It varies with every hour of the day of the season and every season of the year. The designer is concerned with both maximum and minimum rate of flow. The sewers should be designed that it can take the maximum load. At the same time , they should be laid in such longitudinal gradient that there are no deposits in the sewers at the minimum flow. Both the maximum and minimum flows are expressed as percentage of the average rate flow . The maximum daily flow =2 times average daily flow Maximum hourly flow= 1.5 times maximum daily = 3 times average daily The hourly variation of sewage flow in a day follows same pattern as that of hourly variation of water demand but the peaks are low and graphs are flattened due to the reduction in quantity of water and the time required for reaching the pointof gauging. Minimum daily flow= (2/3) x Average daily flow Minimum hourly flow=(1/2) x minimum daily flow