Mixed effects on water quality
Dr Hoi Yeung from Cranfield University's Department of Process and Systems Engineering looks at ways of optimising water quality by improving hydraulics within service reservoirs
Water utilities worldwide are under pressure to meet stringent water quality and supply demands using infrastructure that can be up to 100 years old. Managers must balance treatment and operational objectives in order to ensure adequate supply in the event of unforeseen incidents while limiting the maximum time between abstraction and the point of use.
The demand for public water supply varies seasonally, daily and hourly. Some changes in demand can be accurately forecasted, while others cannot; for example, when thousands of people get up to make tea during the interval of a television
blockbuster. To balance such varying demand would be impossible to achieve even with modern treatment plant control capability and booster pumping stations. In addition, most conventional treatment processes do not respond well to rapid increases or decreases in flow rate. Therefore, treated water storage or service
reservoirs are required in the distribution systems to ensure adequate supply can be guaranteed, even with unforeseen peaks in demand.
A simplified system comprises water abstraction, treatment and distribution stages and this is depicted in Figure 1. Service reservoirs have historically been built for the dual function of maintaining pressure and providing a buffer supply. Existing reservoirs can have storage times of between a couple of hours and several weeks. The resulting degradation in water
quality makes it no longer feasible to focus on point-of- abstraction treatment as a means of assuring all customer and legislation expectations are continuously met.
The problems associated with storage can be classified as chemical, microbiological and physical:
Disinfection by-products that are a major concern are trihalomethane compounds (THMs). This is because chlorine is the most widely used disinfectant as it provides continued disinfection in the form of a residual. As the storage time is increased, the THM formation increases. Therefore large increases in THMs can be associated with service reservoirs with long detention times. The practice of re-chlorination to control biological growth also increases the THM formation. In addition, any increase in pH as a function of storage will result in an increase in the fraction of free chlorine that exists in the form of the hypochlorite ion, which also increases THM formation. Optimising the hydraulic performance of service reservoirs to reduce water age as a method to reduce THMs can be addressed rapidly without major capital investment.
Soluble manganese and iron will often precipitate out of solution and settle in service reservoirs. This occurs as the final treatment process at the WTW and is often disinfected with chlorine followed by pH elevation for distribution. Re-chlorination in distribution accompanied by a pH increase in stored water creates ideal conditions for iron and manganese oxidation. Small concentrations, <100 mg/l, can be sufficient to cause an accumulation of sediment in a storage reservoir over time. A change in the flow pattern in the reservoir or sudden increase in demand in the supplied zone can result in the sediment being re-suspended and entrained in the outgoing flow, resulting in high turbidity, iron, manganese, bacteriological failures, customer complaints and dirty water incidents.
There are many contributing factors that can affect the rate of chlorine decay in a service reservoir. Chlorine is a non-selective oxidant that will react with any organic and inorganic substances in the water or indeed the infrastructure of the distribution system. A long detention time can allow the disinfectant residual to be completely depleted.
It has long been established that chemical decomposition of settled deposits can lead to taste and odours. Chlorine residual can render water more palatable by disguising the underlying taste, which may then become apparent as the chlorine residual decays. The loss of chlorine residual can lead to bacteriological growth that also results in taste and odours.
Strategies which have been adopted to improve bacteriological compliance and water quality at the outlet of service reservoirs include the covering of reservoirs, the binding of soil at reservoir margins, protection against frost and the problems associated with condensation. Engineering improvements such as these have been introduced to
prevent the ingress of atmospheric pollution.
Extensive studies and reviews have shown coliforms could survive and proliferate in areas of very slow moving or stagnating water in service reservoirs. Stressed micro-organisms could recover and re-colonise as the chlorine residual decays.
Where localised fluid velocities in reservoirs are low, particles in suspension such as lime, precipitated metals and corrosion products will settle to form sediments. Not only do these sediments lead to eventual water quality issues when the flow pattern in the reservoir and flow rate changes as previously discussed, they also add a significant operational cost burden in terms of reservoir cleaning.
Reservoir design must deliver its primary function in terms of ensuring security of supply and maintaining pressure. Another priority is that it should maintain the integrity of the incoming water. Table 1 summarises how water quality improvement could be affected by the hydraulic behaviour of a service reservoir.
An ideal service reservoir should:
- ensure no dead areas exist under normal operation,
- achieve good and rapid mixing,
- achieve rapid complete turnover of the tank contents when operating under steady state conditions,
- ensure short circuiting does not occur,
- be hydraulically robust with respect to changes in operation.
In an ideal case, mixing should be instantaneous. In practice, this is not achievable if one relies upon the hydraulic macromixing induced from the inlet flow alone. Achieving good mixing, as opposed to plug flow should minimise the disinfectant losses across the reservoir. Complete and rapid water exchange would limit the formation of disinfection by-products and reduce the risk of taste, odour and de-nitrification. It is important this can be achieved under steady state conditions. Good performance must not only be restricted to a very limited range of operational conditions as these conditions may change with time. An ideal service reservoir should perform well under all conditions.
In reality, there are as many shapes, sizes, inlet and outlet arrangements as the number of service reservoirs. Rectangular (length-to-width ratio up to 4) and circular plan form are the most common shapes. There are some very odd shaped ones. This is because the shape of the land area available could dictate the shape of the reservoir. Figure 2 illustrates just a few possibilities of how single inlet and single outlets of square (in plan) reservoirs could be arranged. It is not difficult to imagine the flow patterns are different for different arrangements.
Figure 3 shows the flow development in a model reservoir with inlet and outlet arrangement as in Figure 2Bw. With this particular configuration, a recirculating pattern is set up in the tank. As the flow moves round the tank, it mixes with the surrounding fluid. However, even after circulating a couple of times around the tank, there is very little mixing with the flow at the centre of the tank. The flow is rather stagnant at the centre. The flow pattern for some of the arrangements could be very complicated. For example, for tanks with multiple inlets, the flow pattern could change when the ratio of flow amongst the inlet varies.
By carrying out scale model tests as shown in Figure 3, the flow behaviour within a service reservoir can be assessed. The degree of mixing and the age of the water in the reservoir could be determined using tracer injection techniques. However, hydraulic modelling has its limits. The size of the model is limited by laboratory space available with 4x2m as a practical maximum. With a service reservoir that could be as large as a football pitch, scale effects could seriously affect the experimental results. Moreover, the time scale between the model and the real reservoir are not the same and thus the chlorine residual within the tank could not be predicted with hydraulic modelling.
Computational fluid dynamics (CFD) can also be used to predict the performance of a service reservoir. Figure 4 is the result of such an analysis at a plane near the water surface. This reservoir has four inlets on the right and the outlet is on the top left corner. The arrows indicate the magnitude and the direction of the flow. In addition to the flow field, the colour shows the predicted level of chlorine residual in the tank (green = high concentration, blue = low concentration). The accuracy of such a prediction will depend on how well the decay of chlorine is modelled. Nevertheless, the result could be used to optimise the design and operation of the service reservoir to ensure good water quality l
(1) O’Neill, S. Hydraulic optimisation of Service Reservoir to maintain water quality in distribution systems, Eng D Thesis, Cranfield, 2001.