Water Treatment Plant Working Principle

WATER TREATMENT PLANT WORKING PRINCIPLE

1.

PROCESS DESCRIPTION

1.1

INTRODUCTION

Most of the ground waters in Southern Nigeria have elevated contents of iron, manganese and free (corrosive) carbon dioxide, and thus, they are not suitable for human consumption. The proposed water treatment plant is capable to reduce the contents of iron and manganese and to increase the pH-value to acceptable values for potable water by aeration, flocculation and pressure filtration. The filtration velocity of this plant is 10.6 m/h at the nominal capacity of 12 m3/h.

1.2

WORKING PRINCIPLE

The corrosive carbon dioxide is reduced to tolerable values by spraying of the raw water into the aeration chamber of the three chamber raw water treatment tank. Normally, there is no need for a pH correction if the raw water is well aerated. Most of the free CO2 which is responsible for the low pH value is degassed during the aeration and the pH value thus rises without any addition of chemicals to values ranging between pH 6.5 and 7 which are perfectly acceptable by all standards for drinking water. The efficiency of chlorine as sterilising agent is still high at pH 6.5 to 7. At the same time, the water is aerated in order to oxidise the iron (Fe++ to Fe+++), which is necessary for the coagulation and flocculation of the iron hydroxides (Fe+++) for their posterior removal in the process of pressure filtration. The oxidation of Fe++ to Fe+++ is

essential because Fe++ salts are soluble and Fe+++ salts are insoluble and can thus be retained after coagulation / flocculation by the filter media of the pressure filters. Furthermore, Calcium Hypochlorite (HTH) solution is injected by a chemical dosing pump as sterilising agent. The aerated raw water passes from the aeration chamber through an underflow into the settling chamber and thereafter through an overflow into the pump chamber. During the stay in the settling chamber, iron Fe+++ hydroxides together with other suspended solids will continue to coagulate and to form flocs. Part of the iron hydroxides and suspended solids are settling in this tank. This sludge precipitate has to be removed from time to time.

2 Aluminium sulphate is commonly injected in water treatment plants as a flocculating agent, but we know by experience that the groundwaters from deep wells in Lagos do not need the addition of flocculating agents. Aluminium sulphate is mainly used as a flocculating agent in surface waters with brownish colouration deriving from swampy areas containing humic acids, but not so much in ground waters from deep wells. The handling of aluminium sulphate in very soft waters requires a lot of experience and continuous control. The groundwaters from deep wells in Lekki are extremely soft. The pre-treated raw water is pumped by one of the transfer pumps from the pump chamber through the two pressure filters into the underground clean water storage tank. The flocs which have been formed during the pre-treatment in the raw water tank and which have not settled in the raw water tank are retained by the filter media of the pressure filters. When the filter media is saturated, it has to be regenerated by backwash. The system works by automatic controls. A float switch on the pump chamber of the raw water tank controls the submersible pump of the borehole. A second float switch is installed on the raw water tank as dry running protection for the transfer pumps. The chemical dosing pump injecting Calcium Hypochlorite (HTH) into the raw water line and the electrical agitator on the chemical storage container are controlled by the submersible pump of the borehole. The adjustment of the dosing system has to be done manually and only while the dosing pump is working! Please see the attached flow chart of this water treatment plant.