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Water purification

Efficient water purification is essential even in most big cities through out Africa. The potential risk comes from the many different species of protozoa, bacteria, cysts and viruses, which cause a broad range of diseases, such as diarrhoea, dysentery or cholera… Certain bugs such as the giardia and cryptosporidium are resistant to most of the chemical treatments, and have to be filtered with 0.45 microns filters, while most viruses are smaller than the mesh of the filter and have to be killed with suitable chemicals. A combination of mechanic and chemical treatment is adapted for a correct water purification, as described in the following 3 different steps:

Coagulation

The coagulation aims to form big particles, or flocs, of clay, other minerals and organic matter, which are the main sources of turbidity in the water. After a while, the flocs settle in the container and are easily separated from the water. This primary treatment roughly clears the water before the filtration step, avoiding the fast clogging of the expensive ceramics and membrane filters. Many different aluminium or iron salts or polymers are used in industry for coagulation purposes, and the aluminium sulphate is the most common of them.

The single most important variable for coagulation is the pH, which influences the flocs' size as well as the quantity of coagulant to use in order to have the best results (usually between 15 to 100mg/l of aluminium sulphate). Paradoxically, the higher the turbidity, the smaller concentration of coagulant, because of the high collision probability of the particles. The organic matter in suspension is usually adsorbed onto the mineral particles, and its concentration does not influence the coagulant demand. The coagulation is not instantaneous, and time is required to have the maximum enlargement of the flocs' size, and for the decanting of the particles formed.

A 5% solution of Aluminium Sulphate (ex: 25g/500ml) is prepared and can be kept for the duration of the journey (solution very stable). For the coagulation treatment, 0.2 to 1ml of this solution per litre of water are dropped with a pipette. Stir it up and let the coagulated particles settle for 30 min.

Note: the pH of the raw water has to range between 6 and 7.8 in order to achieve the best coagulation with the smaller amount of the Aluminium Sulphate (reducing consequently the residual coagulant concentration in the effluent). Alkaline pH are adjusted with addition of concentrated hydrochloric acid.

Filtration

If the water is still turbid after the coagulation, used coffee filters pre-filtrate the bigger particles, and filters are changed when needed (the nylon filters, which can be used indefinitely after cleaning are very useful for this operation).

Afterwards, 0.45microns Miniworks filters or the membrane filtration unit filter organic matter left in the water. When the flow rate becomes too low, the membranes are changed, or clean the ceramic of the Miniworks is cleaned.

Important: the volume of water possibly filtered by the Miniworks or the membrane depends largely on the water quality, coagulation and pre-filtration stage are therefore essential for a good use of the filters when the quality of raw water is poor.

This 0.45microns filtration stage should filter most organic matter contained in the water, and is an efficient way to get ride of cryptosporidium and giardia cysts. The 0.45microns filtration does not eliminate the viruses from the stream!

Disinfecting

The safest way to sterilise water is to boil it during at least 20min (some bugs are quite resistant to high temperatures!), but this operation is time and energy consuming and not suitable for a day to day use. Some solar distillation units developed in African countries also are very efficient, but not easily transportable.

Several chemicals are used for disinfecting the water. The most efficient is the colloidal silver, which kills more than 650 types of bug, including the cryptosporidium. Colloidal silver is found in the Micropure tablets, which are very efficient but expensive for long term use. Decolourised iodine tincture is also widely used (4drop per litre, and action time 30min) but can create allergies to certain persons and has to be used cautiously.

Chlorine is the cheapest way to kill viruses and has the particularity to have a long term disinfecting effect if the water is stored in a tank sealed from atmosphere (so the chlorine does not evaporate). 3.5 or 5% standard bleach solution (sodium hypochlorite) can be used. Be aware of phosphates and different chemicals added. The bleach solution cannot be kept a long time and starts degrading as the bottle is opened.

The solution chosen for the purposes of the trip is the HTH for swimming pool, as it has been approved as a NSF60 and its use is suitable for large scale water treatment. Its solid granule shape, concentrated in active chlorine and its stability make it easy for transportation and efficient to use. It is nevertheless very flammable when in contact with organic matter such as oil and petrol, and therefore will be kept far from lady blue energy drinks. HTH for swimming pool is a solid form of calcium hypochlorite (68% in mass as CaOHCl), which produces 99.2% of active chlorine. The composition balance is mainly composed of water (5%), CaOH (2%), calcium chlorite (2%), sodium chlorite (15 to 20%) and sodium hydroxide (1%) and contains no harmful chemicals when used in diluted solutions. Solution at 15g/l (1% chlorine) are made and stored in non-metallic containers, air tight and opaque. Otherwise, the solution starts to degrade, and the chlorine is release as Cl2, or reacts with UV light. During the trip, the solution is kept for a maximum of 10days before being replaced to keep a maximal efficiency.

For a relatively clear water, the chlorine treatment applied ranges between 5 and 10mg of chlorine per litre of raw water (=around 2.5ml of the HTH 1% solution), and increases with the turbidity of the water. It is nevertheless important to remember that the clearest the water, the more efficient the disinfecting. The residual free chlorine left after 30min (contact time of the chlorine) must be superior to 0.5mg/l (use the DPD1 free chlorine swimming pool test kit) to ensure the disinfecting. The water can then be kept in a sterilised tank for a couple of days (test the disinfecting level with DPD1 before drinking it if stored for a long time). Note that for very alkaline waters (pH above 8.5, tested with the swimming pool test kit), the disinfecting is less efficient, and the contact time of the chlorine can increase from 12 to 24hours.

Note: never pour raw water in the container used for the storage of the disinfected water, otherwise a contamination is possible. Use different drums and containers for the different stages of the treatment and keep all the instruments clean.

Water Kit