In Scotland, the organisation responsible for maintaining the supply and sanitation of fresh water is Scottish Water (SW). The body in charge of ensuring adherence to environmental regulations is the Scottish Environmental Protection Agency (SEPA).
In turn potable (drinking) water standards as well as discharges of effluent are set by the EU. Within the standards set by European-wide water management policies, SW operates the following infrastructure:
- 300 water treatment works as well as ancillary facilities such as pumping stations, reservoirs and sewage sludge treatment centres.
- Over 1800 waste water treatment plants, including 1200 septic tanks.
- Approximately 50,000 km (31,000 miles) of drainage pipework.
Water treatment is concerned with the entire set of processes that produce water that is fit for a particular purpose, this may not mean for human consumption. Waste water treatment is distinct from this definition because it is concerned with the collective set of physical and biochemical techniques employed to ensure that water is specifically safe for human consumption.
In addition, waste water treatment is normally associated with processing water that has been utilised in some way, for instance in an industrial process or is actually sewage.
Why treat the water?
Most of water that comes from the taps of Glasgow and Scotland is sourced from reservoirs, lochs and river systems. Although not likely to be as polluted as effluent water, this fresh water is unlikely to be pure and so it must be treated, it is likely to contain substances such as:
- Bacteria and other microorganisms, some of which may be Pathogens.
- Dissolved metal ions such as Calcium (Ca2+) and Magnesium (Mg2+) and carbonates thereof, dissolved from the rocks.
- Dissolved non-metal ions such as Nitrates (NO3–) and phosphates (PO43+)
- Solid material and substances such as mud, sand, grit and organic detritus
- Colloidal clay and other soil particles
- Objects dumped by people, this is in effect pollution.
The EU regulations mentioned above require Scottish water to carry regular sampling of the sources of water which they use to supply the drainage system of the country, a remit which also covers ground water and aquifers. For example the microbes that cause water-borne diseases such as cholera (vibrio cholerae) and amoebic dysentery (Entamoeba histolytica), must be removed.
Colloidal clay occurs when the particle is dissolved within another molecule or substance and so cannot be removed by those techniques designed to remove suspensions. Colloids in their entirety are removed by adding coagulants such as aluminium sulphate (Alum Al2(SO4)2). Nitrates are particularly toxic to infants and young animals and so there level is kept below 50mg/l.
Water Treatment Techniques:
A stringent sampling regime therefore ensures that the treatment techniques are effective; the water is treated in the following stages:
- A large mesh screen is used to remove any large natural objects as well as any that are dumped in the water course.
- A second, finer mesh traps any larger particles of solid material. The water is then passed through a layer of sand or powdered and activated anthracite, (coal of the highest carbon content).
- The water is then aggregated (or coagulated) so that all solid particles (such as colloids and suspended material) are removed. Substances such as alum bind the particles together forming clumps in a process called solid-liquid separation. The pH (acidity) of the water must be kept between 5.8 and 6.5 (slightly acidic). The alum will increase acidity and so must be added in measured amounts and the water itself be well mixed.
- While the water is being coagulated, activated organic (carbon based) synthetic (derived from oil) polymers are added in measured amounts. The polymer molecules promote further clumping (aggregation) thus increasing the size of the coagulating particles. This process is called flocculation, and the type of polymer used will depend on the overall quality of the water itself.
The material formed as a result of these complex chemical reactions form a surface layer of sludge. The sludge is removed, and the remaining water is then allowed to settle in a process called clarification that occurs in sedimentation tanks.
Clarification and other processes
Clarification refers to a set of chemical techniques which are applied to the water during and after it has undergone coagulation and flocculation. For example, dissolved air flotation occurs when a high-pressure stream of very fine bubbles is blown into the flocculated water. This forces any remaining suspended material to the surface where it is continually skimmed off.
Simultaneously, at very high speed the water is passed through another set of sand or carbon filters. This rapid gravity filtration ensures that any other suspended particles are extricated from the system. The water is often sampled at this stage and if necessary recycled back into the system for secondary treatment. Once it is of sufficient purity any microbes, pathogens and dissolved ions are removed. In the latter case, PO43+ ions are removed by the controlled addition of powdered iron fillings.
If necessary, NO3– ions are removed by specialist nitrifying bacteria that convert the ions to nitrogen gas. In some cases, specialist ion exchange techniques are used to eliminate these and other types of inorganic ions. Further filtration removes any additional sludge material.
Disinfecting the water
Disinfection is only effective if the maximum possible quantities of suspended and colloidal materials have been removed. This includes natural organic compounds dissolved in the water that have a tendency to react with chloride (Cl–) ions forming a class of compounds called tri-halo-methanes (THM). These form when three of the hydrogen atoms in a molecule of methane (CH4) are substituted for chlorine atoms.
The THM’s cause discolouration of the water and if consumed can cause damage (amongst other undesirable effects) to the renal organs. Micro-organisms (beneficial or pathogenic) are removed by bubbling ozone (O3) through the water or more commonly chlorine, or compounds that contain chlorine are added. The chlorine dissolves in the water forming a substance called hypochlorous acid, which can pass through the membranes of both bacteria and water borne disease microbes. Once inside cell or microbe the hypochlorous acid reacts with its constituent proteins thus overwhelming its metabolic processes. To be effective, the water must be kept alkaline between pH 7.2 and 7.8. This treatment is analogous to the addition of chlorine to swimming pools and other water recreational facilities.
Overall, water treatment is a complex series of mechanical, biological and chemical steps that must be carried out in a precise order and with great care and attention. In addition, many of the techniques have been in operation for over a century and are being continually improved, irrespective of the already strict legislation.