Drinking Water Safety (Part 1)
This article was originally printed in Louisiana Civil Engineer magazine, May 2011, Volume 19, No. 3.
As engineers, we are generally concerned with design and construction issues on our engineered projects, with little feedback on their operations – unless there are problems! In essence, our project may become a “one and done” event never to be seen or thought about again. In the spirit of protecting the public health, let us consider the safety of the potable water supply. Regulations are in place to ensure the delivery of clean drinking water to all customers of public water systems. It is the responsibility of the water purveyor (supplier) to abide by these regulations. Operational procedures are in place for water system operators to monitor the water quality. However, once the potable water exits the water purveyor’s distribution system, water quality monitoring is essentially non-existent. Typically, the water purveyor’s distribution system terminates at the customer’s water meter where the purveyor losses control of the water quality. Protection of the water supply becomes the responsibility of the customer once the potable water has passed through the water meter and, is therefore, no longer the responsibility of the water purveyor. Well … that may not necessarily be the case!
Consider for a moment that a cross connection (any physical connection or arrangement between two separate piping systems, one of which contains potable water and the other, water of unknown or questionable safety, whereby water may flow from one system to the other, the direction of flow depending on the pressure differential between the two systems) on the customer’s property and there is a loss of pressure on the supply side (purveyor’s supply). An example of a cross connection is shown in Figure 1.
As an example, water may be supplied to customers at 70 psi; however, actual pressure on the customer’s property may be at 55 psi (due to losses). Basic hydraulic principles state that water will always flow from high pressure to low pressure. With supply side pressure failure, the customer’s plumbing essentially becomes the supply side forcing the potable water to backflow (the flow of water or other liquids, mixtures, or substances into the distribution pipes of a potable supply of water from any source or sources other than its intended source) into the purveyor’s water system. If an active cross connection is in place at the time of the purveyor system failure, then the likelihood of contaminants entering the public drinking water system is very real and could be hazardous – or even deadly. Numerous case histories exist that document backflow incidents. (A Google search of “backflow incidents” yields approximately 120,000 results).
Backflow is classified into two types: backsiphonage and backpressure. Backsiphonage occurs when there is a loss of supply side pressure. Examples of backsiphonage are a broken water line or a draw on a fire hydrant when fire fighting activities are in place. When water is removed from a hydrant with a pump, a vacuum may be created on the system and backsiphonage backflow may occur – much like sucking on a straw! Backpressure backflow may occur when the customer side pressure exceeds the supply side pressure. Typically this occurs with a pump on the customer’s premises.
As engineers, we may be familiar with the regulation of The Safe Drinking Water Act of 1974. The regulations establish the maximum contaminant levels (MCL’s) in the potable water supply. These regulations stipulate the amount of various biological and radiological chemicals and byproducts that can be present in the potable water are under seemingly constant revision and expansion. Water system owners and operators are well trained in the operation of their systems and the vast majority provides outstanding water to their customers. Sampling at various locations throughout the water system ensures that a quality product is being produced and delivered to the customer, all in accordance with Federal and State regulations. In Louisiana, the State regulatory agency is the Louisiana Department of Health and Hospitals (DHH). DHH regulations on cross connection control and backflow prevention have been in place for many decades; however, enforcement efforts have recently intensified to require water systems to implement a backflow prevention program.
DHH monitors a system’s backflow prevention program through sanitary surveys – periodic evaluation of a systems conformance to the rules and regulations. A system found in non-compliance will likely be sited with a “deficiency” that must be addressed by the purveyor of the water system within a specified time frame. DHH now considers these deficiencies “significant deficiencies” and failure to correct these will first result in a “treatment technique” violation that requires public notice! Should the system still not comply, then ultimately an Administrative Order will be issued and the system subject to a fine of $3,000 per day per violation. The goal of DHH is not to be over regulatory but to insure the safety of water system customers. It is likely that very few individuals working outside of the water industry ever consider the quality of the water they drink. Customers expect clean, safe water from the purveyor and that responsibility falls on the municipal engineers, regulatory officials, and operators of the system.