Q.
Explain any three (3) of the following:
i) Combine System in underground sewerage system.
ii) Back Siphonage in water supply system.
iii) Factors to be considered in selecting sanitary appliances.
iv) The location of manhole in the sewerage system.
(25 marks, 2015 Q7)
A.
In Part 1 D06 BT1 there was a question on underground discharge. Another similar question was asked in D12 2012 Q7 on Separate System. See the difference in 2012 Q7 at the link here.
Types of Discharge at Construction Site. And, this is from different chapter different from sewers and sanitary water management.
(i) Combine System in underground sewerage system
Over flow of the combined sewerage system
The advantages and disadvantages of this combined sewerage system are listed below.
Ref:
Various sources - mainly
Solomon Seyoum. Types of Sewer Systems. Available at
http://ocw.unesco-ihe.org/pluginfile.php/440/mod_resource/content/1/Urban_Drainage_and_Sewerage/1_Introduction/Types%20of%20sewer%20systems/Type_of_sewer_systems.pdf
(ii) Back Siphonage in water supply system.
Backflow and Backsiphonage
What is backflow?
Backflow means the reversal of water flow from its normal or intended direction of flow. Whenever a water utility connects a customer to the utility's distribution system, the intention is for the water to flow from the distribution system to the customer.
However, the flow of water could be reversed from the customer back into the distribution system. If cross-connections exist within the customer's plumbing system when backflow occurs, then it is possible to contaminate the public water supply. There are two types of backflow -- backpressure backflow and backsiphonage.
What is backpressure backflow?
Backpressure backflow occurs when the pressure of the nonpotable system exceeds the positive pressure in the water distribution lines; that is, the water pressure within an establishment's plumbing system exceeds that of the water distribution system. For example, there is a potable water connection to a hot water boiler system that is not protected by an approved backflow preventer. If pressure in the boiler system increases to a point that it exceeds the pressure in the water distribution system, a backflow from the boiler to the public water system may occur.
A downstream pressure that is greater than the potable water supply pressure causes backpressure backflow. Backpressure can result from an increase in downstream pressure, a reduction in the potable water supply pressure or a combination of both. Boiler pumps, pressure pumps or temperature increases in boilers can create increases in downstream pressure. Reductions in potable water supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting or breaks in water mains.
What is backsiphonage?
Backsiphonage occurs when there is a partial vacuum (negative pressure) in a water supply system, which draws the water from a contaminated source into a potable water supply. The water pressure within the distribution system falls below that of the plumbing system it is supplying. The effect is similar to siphoning or drinking water through a straw. For example, during a large fire, a pump is connected to a hydrant. High flows pumped out of the distribution system can result in significantly reduced water pressure around the withdrawal point. A partial vacuum has been created in the system, causing suction of contaminated water into the potable water system. During such conditions, it is possible for water to be withdrawn from nonpotable sources located near the fire -- for example, air-conditioning systems, water tanks, boilers, fertilizer tanks and washing machines -- into buildings located near a fire. The same conditions can be caused by a water main break.
Garden hoses, toilets or similar devices create most household cross-connections. Under certain conditions, the flow in household water lines can reverse and siphon contaminates into the water supply. A toilet installed incorrectly without a "plumbing-code approved" toilet ballcock (air gap) will allow contaminated water to backflow to other water outlets in your house, including the kitchen sink.
What is a Backflow Preventer?
A backflow preventer is a method or mechanical device to prevent backflow. The basic method of preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. Mechanical backflow preventers are devices that provide a physical barrier to backflow. There are four devices commonly used -- the reduced pressure principle assembly, the double check valve assembly, the pressure vacuum breaker and the atmospheric vacuum breaker. All of these devices require periodic maintenance and testing.
Ref:
http://water.ky.gov/DrinkingWater/Pages/BackflowBacksiphonage.aspx
iii) Factors to be considered in selecting sanitary appliances.
Sanitary wares are the most useful and basic elements in a bathroom. When choosing the best sanitary ware for your bathroom, your need to consider several factors for example t make and the manufacturing company for example Twyford bathroom accessories.. They should be practical to use as well as be pleasant to look at. Therefore tactical knowledge will help you make good decision when you go to purchase your bathroom sanitary ware. Some of the factors are highlighted below.
Ease of cleaning
When buying a sanitary ware you should consider the amount of cleaning required in order to maintain them. When abrasive solvents and breaches are frequently used on the sanitary wares, their surfaces become hoarse and over time the blaze of the product is affected. This makes them susceptible to dirt and dust and reduces their smoothness.
Resistance of deformities
While choosing bathroom sanitary ware, two major factors have to be put into consideration. These are the strength and the design. The toilet bowls and bathroom basins should be of a good design and also be strong enough not to chip and break easily. They should therefore have no sharp edges and you should ensure they are well fitted.
A style that matches your home decor
A style that matches your home décor also will influence your choice. In a traditional home, you will have to consider marble and the antique as these are the available elegant vanities. A modern bathroom vanity will best fit a contemporary home style.
A good bathroom reflects an individual’s persona. Bathrooms with good accessories give one vigour and comfort. Therefore the sanitary ware in your bathroom needs to consider as one does the purchase. Good choices have to be made as one furnishes the bathroom. There is therefore need for designer sanitary ware in the bathroom.
How to Choose Bathroom Sanitary ware. Posted on March 4, 2013. The Need For Designer Sanitary Ware in The Bathroom. Available at,
iv) The location of manhole in the sewerage system. More about manhole and diversion manhole here.
Manhole is provided when
- There is change in grade of sewer
- There is change in alignment
- There is change in size of sewer
- At junction of two or more sewers
- Manhole is also provided in straight alignment of sewers at regular intervals depending upon the diameters of sewers. It ranges from 90m to 150m (300' – 500') e.g. 75m for 60cmф, 120m for 90cmф and 150m for 120cmф.
Types of Manhole
Depending upon the depth the manhole can be classified as;
1. Shallow manhole
It is provided at shallow depth of 75-90cm (2'-3'). It is provided at the beginning of branch sewer or at a place not subjected to heavy traffic. It is provided with a light cover at its top it is also called inspection chamber.
2. Normal Manhole
It is provided in sewer line at depth of 150cm with a heavy cover on its top. It is generally of square shape (or rectangular shape).
3. Deep Manhole
They are provided at depth greater than 150cm with heavy cover at the top. The size is gradually increased and a facility for going down is provided.
Ref:
FOCUS, Manhole, available at,