These questions and answers focus on the basics of water supply operations and the distribution system. Distribution maintenance includes routine sampling, inspection, leak repairs, maintaining carrying capacity, pipeline cleaning, lining mains in place, main-break repair and thawing of frozen pipelines. These questions and answers are from the book Introduction to Water Distribution, Principles and Practices of Water Supply Operations, Volume 3, AWWA, Denver CO, 1986, ISBN 0-89867-188-4.

Question 1:  What type of distribution system configuration is not recommended and why?
Answer:  The tree-type system is not recommended because only a few loops are formed, resulting in the creation of several dead ends. Circulation becomes restricted, making it difficult to supply a continuous flow of good quality water to all parts of the system.  Two preferred distribution system configurations are the arterial-loop system and the grid system. 

Question 2:  What is a water hammer and why is it important in distribution system operations?
Answer:  This is the shock or pressure wave that travels down a pipe due to rapid opening or closing of valves or sudden starting or stopping of pumps. The increased pressure can cause extensive damage to pipes and fittings. Water hammer is also a surge, which is a momentary increase in pressure. Sudden changes in velocity will cause a surge.

Question 3:  What is meant by the C-value?
Answer:  The C-value or C-factor is a hydraulics equation solving for the friction coefficient. It is an indication of how easily water can move through a pipe. The higher the C-value, the smoother the interior of the pipe and the easier it is for water to flow. The C-factor is related to the roughness of the pipe's interior surface, which can affect the pipe's carrying capacity. It is referred to as the Hazen-Williams C-factor prediction model. Programs like Water PIPER (W-PIPER) help make cost-effective decisions for underground water distribution systems based on Hazen-Williams C-factors and the life of repair alternatives.

Question 4:  List four types of corrosion that can occur in a distribution system and summarize the causes of each.
Answer:  Corrosion is the gradual destruction or deterioration of materials by chemical action. It is frequently induced by electrochemical processes. Four types of corrosion are internal electrochemical corrosion (e.g., aggressive water), external corrosion (e.g., soil conditions), galvanic corrosion (e.g., dissimilar metals) and stray-current corrosion (e.g., uncontrolled DC current).

Question 5:  Explain what causes tuberculation and what effect it has on the distribution system.
Answer:  Tuberculation refers to the growth of nodules (tubercules) or knobs of rust on the pipe interior. It is caused by the deposition of corrosion products on interior pipe walls. This results in roughness, which reduces a pipeline’s carrying capacity.

Question 6:  When should mains be flushed? 
Answer:  The most common practice for maintaining or improving the system carrying capacity and water quality is flushing. Mains should be flushed prior to disinfection of mains in order to remove any foreign material that may interfere with disinfection or reduce water quality. Any new or repaired water main must be thoroughly cleaned (flushed), disinfected and tested for bacteriological quality before it can be put into service. This can be done after pressure and leakage testing. Flushing at high velocities at least twice a year in problem areas will normally remove most settled substances and stale water. Recommended velocity to obtain proper flushing action is 2.5 fps (0.8 m/s). Flushing the system late at night will achieve greater flows through the line and cause fewer customer complaints. Hydrants must be fully opened when flushing. Open and close hydrant valves slowly to prevent water hammer. 

Question 7:  List and describe three methods of disinfecting water mains.
Answer:  (1) Continuous-feed method, e.g., adding chlorine at a constant rate while the main is filled.  (2) Slug method, e.g., high rate of chlorine is applied to a slug of water moving through the main. All parts of the main should be exposed to at least 100 mg/L chlorine for three hours. (3) Tablet method, e.g., placing calcium hypochlorite tablets in the main as it is installed. The main is filled with water and allowed to stand for at least 24 hours. 

Question 8:  What is the purpose for maintaining a chlorine residual?
Answer:  Maintaining a minimum disinfectant residual prevents microbial growth in the distribution system. Referred to as "biological stability," this provides a condition where the treated water quality does not enhance biological growth in the distribution system. Water systems that use chlorine for disinfection must report daily free chlorine residual readings on their Monthly Operating Report (MOR) and maintain a minimum free chlorine residual level of 0.2 mg/L (401 KAR 8:150). For water systems using chlorine, the minimum requirement in the regulations is to report free chlorine residual; however, for operation and maintenance, the system could, periodically, report total chlorine for comparison of total to free chlorine.

Question 9:  Explain the importance of as-built plans (conforming to construction records).
Answer:  These illustrate how the mains, valves, hydrants and other connections were actually installed. The records are important since final routes and installation will most often differ from the original plans.

Question 10:  Explain why routine water-quality monitoring of a distribution system is important.
Answer:  It helps ensure that high quality water is being delivered to the consumer. These practices can also provide information concerning the functioning of the treatment plan, the condition of the distribution system and whether contamination is entering the system.

Question 11:  Indicate why a routine hydrant flushing program is considered to be good operating practice.
Answer:  It helps rid the pipeline of sediment, slime and corrosion by-products, which can adversely affect water quality and the carrying capacity (C factor) of the pipe. A flushing program also allows routine maintenance to be performed on hydrants.

Question 12:  Explain the term "unaccounted-for water" and its importance to management and operations.
Answer:  This is the difference between the amount of water pumped into the distribution system and the amount billed to the consumer through a metered system. It represents lost revenue to the utility and is an indication of problems associated with leaks and under-registering of meters.

Question 13:  What are two basic methods for conducting leak surveys?
Answer:  (1) Listening with either mechanical or electrical instruments or (2) conducting a water audit.

Question 14:  Name a computer program that can model and simulate hydraulic and water-quality behavior within pressurized pipe networks.
Answer:  EPANet or Pipe2000 (KYPipe).  See this page for a comparison.