Three-step system

Function

Step 1 – River water

In the first step (1) water is pumped to a 1000 litre bucket from the tributary. A tributary is to prefer instead of the main river because contamination is likely to be less here. Also there are no villages located upstream Puerto Triunfo along the tributary which makes it more preferable. Water is pumped with a gasoline driven motor pump, and is transported in plastic tubes (40 millimeter in diameter) approximately 250 metres to the location of the plant. The plastic tubes are placed in the ground.

Step 2 – Rapid mixing
The stair

The second step is a stair (2) made of wood. This stair is sealed with tar, and meassures about 4 metres from the first bucket to the second. It is 35 centimetres wide, has 5 steps and a fall depth of approximately 1 metre between the buckets. The stair also has got walls to avoid unneccesary waste of water. These walls meassure a hight of about 35 centimetres.

Water is let out from a tap with a diametre of 7.5 centimetres, which is placed close to the bottom of the first bucket. The tap should be completely open when the stair is in use. It is important that all water is transported through the stair within 90 seconds.

Chemicals

Chemicals are added in the very first step of the stair, and the stair is inteded to give a very rapid mix between water and chemicals. The chemical used is Sulphate Aluminium Type B. 75 grams of dry Sulphate Aluminium is mixed with clean water to a total weight of 1500 grams (to a 5% product solution). This mix of chemical and water is distributed evenly in the 1000 litres of water passing the stair. It is important to remember that this mix and the amount of 1500 grams only is suited for cleaning the amount of 1000 litres of water from this certain tributary.

Step 3 – Sedimentation

After the stair, the water arrives to the second bucket (3). In this bucket it is important that the water moves around constantly for about 20-30 minutes, but very, very slowly. The water is given a small speed from falling from the stair, but if it slows down to much, a small pole could be used to stir and maintain the motion.

The aim of this step is to create sedimentation, that is, to let the dirt particles in the water fall to the bottom. Sedimentation facilitates the following filtration step by reducing the amount of particles to be filtrated. This also makes the filter more durable. The chemicals helps the unwanted particles in the water to flocculate (connect to each other). These flocks are heavier than the small particles and therefore falls to the bottom easier. The bigger flocks, the faster sedimentation. The reason why it is so important not to stir to fast and to hard is that this could brake the flocks very easy.

Step 4 – Filtration

Water is in this step filtrated through sand. This sandfilter is built in a bucket (4). At the bottom two layers of gravel are placed, 10 centimetres each, starting with the coarsest at the very bottom. On top of the gravel there is a sand layer, approximately 1 metre thick. When water is let through the sand filter, the sand is at first cleaned. After a while a micro flora is built up in the upper part of the sand, about 10 centimetres. This micro flora is reducing the bacterias in the water, and is very important. It is therefore important to avoid stiring in the sand, othervise the layer might be disturbed. The water also should be spread evenly over the sand filter, to avoid high pressures from the water, which also could destroy the micro flora. It could take as much as a month before a good micro flora layer is built up. The micro flora can not survive without a constant flow of water through the sand. Therefore, it is important to always keep water in the filter, and never let the sand dry.

Step 5 – Clean water

From the sand bucket the water is let out to the last bucket (5), where the water is supposed to be good enough to drink.

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All text above is quoted from following essay:

Andersson, S., & Erlandsson, K. (2006). Water treatment Puerto Triunfo, Colombia . Bachelor, Uppsala University, Aquatic and environmental engineering, Uppsala.