Editor-in-Chief Elisabeth Lisican showcases a handful of features to read in the April 2017 issue of Water & Wastes Digest.
The quality of life for Americans was greatly enhanced in the early 1900s due to the introduction of chlorinated drinking water throughout the U.S. water systems. From the stockyards of Chicago to the Boonton Reservoir of Jersey City, the case was established that filtration alone was not sufficient to guarantee clean water.
By the 1920s, chlorination was well entrenched as the primary means of disinfecting drinking water. The combination of filtration and chlorination reduced typhoid fever by 91% within five years, which led to its near eradication by 1936, according to a statistical study of disease rates.
Chlorine-based disinfectants have been the choice for treating drinking water since the turn of the 20th century. This is the only type of disinfectant that provides a residual in the distribution system that is vital to preventing waterborne diseases. Three forms of chlorine that are commonly used are gaseous chlorine, calcium hypochlorite tablets and sodium hypochlorite solution.
However, safety concerns and costs have prompted many municipalities to switch from chlorine gas to sodium hypochlorite. The most common disinfection method is some form of chlorine or its compounds such as chloramine or chlorine dioxide. Other nonchlorine-based disinfectants are ozone and ultraviolet radiation.
According to the Water Quality and Health Council, 98% of systems that treat water utilize chlorine-based disinfectants. Moreover, the World Health Organization concurs that disinfection by chlorine is still the best guarantee of microbiologically safe water.
The Orange Water and Sewer Authority (OWSA) operates the Jones Ferry Road Water Treatment Plant in Chapel Hill, N.C. The plant serves as a model for state-of-the-art improvements. The plant can treat up to 20 million gal of raw water daily—nearly double the average demand of water usage from the local population of 70,000 people.
The water treatment process encompasses several phases. The first phase is the addition of powdered carbon to the water supply from the lakes to improve the taste and control odor in the water. Secondly, the solid particles are separated from the water in settling tanks. Once that process is complete, the water is pretreated with sodium hypochlorite and then filtered through layers of sand and anthracite coal. Lastly, chemicals are added for disinfection and public health. The chemicals include chlorine, ammonia and fluoride.
The pretreatment stage consisted of a bank of 10 diaphragm pumps injecting sodium hypochlorite continuously and simultaneously at a rate of 1.3 gal per minute against zero back pressure. OWSA noticed numerous problems with the diaphragm pumps in this particular application. The central issues were the outgassing or off-gassing of the sodium hypochlorite and the inability of the diaphragm pump to operate efficiently due to the gas buildup in the pump head. Enviably, a loss of prime would occur, costing OWSA valuable downtime.
Despite the fact that auto-degassing valves were implemented, the problem was not totally eliminated. Consequently, the treatment plant’s chief operator, Dusty Martin, was compelled to make the switch to peristaltic pumps.
First and foremost, the peristaltic pump eliminates the core problem with outgassing of the sodium hypochlorite by utilizing precision-engineered rollers to optimally squeeze a heavy-duty norprene tube efficiently and with unparalleled accuracy. Gas has no effect on this simplistic pumping method. Furthermore, the tube life has been greatly increased through an innovative rotor design that allows the pump to handle the same maximum pressure in either direction. The ability to reverse the motor, in essence, doubles the tube life. Of course, the tube life will always vary depending on the chemical used, the output pressure, the size of the tube and the rpm of the motor.
Other notable advancements with the peristaltic pumps include a tube failure detection system built into the pump head; increased output pressure of up to 125 psi; precision stainless steel ball bearings supporting the front and back of the rotor shaft; and highly advanced electronics to connect to SCADA systems.
At the Jones Ferry Road plant, the peristaltic pumps are wired via 4-20mA into the SCADA system for monitoring and alarm responses. Martin said that he was amazed how easily the peristaltic pump hooked up into the plant’s SCADA system. Continual monitoring using powerful graphic and alarm software programs allows the SCADA system to ensure the exact amount of chemicals have been dispersed into the water supply.
In addition, Martin said that the dosing is much more precise, accurate and consistent using a peristaltic pump compared to a diaphragm pump. Overall, Martin highly recommends a peristaltic pump over a diaphragm pump for sodium hypochlorite injection.