Thesis Abstract

Aluminium Sulphate (Alum) and ferrous sulphate, being among the best known coagulants in water treatment were investigated with the aim of comparing their coagulation efficiencies. The parameter levels of a turbid water were determined before and after treatment with alum and ferrous sulphate respectively at coagulant dosages of 1 to 10g per 3 litres of turbid water, for each of the following parameters pH, total suspended solids (TSS), dissolved oxygen (DO), biochemical oxygen demand (BOD5), turbidity, chloride, fluoride, phosphate and chemical oxygen demand (COD). The turbid water was prepared by addition of ground humus soil to tap water. Using a sedimentation beaker, experiments were conducted, leading to optimum coagulant dosage in some parameters. Coagulation experiments of the turbid water at the coagulant dosage of 10g per 3 litres, gave the following coagulation efficiencies with alum as the coagulant pH (44.92%), TSS (98.71%), DO (90.10%), BOD5 (100%), Turbidity (98.70%), chloride (100%), fluoride (100%), Phosphate (80%), COD (100 %) and Copper (0.00%). Similarly, using ferrous sulphate coagulant for the same level of turbid water and same dosage, the coagulation efficiencies achieved were pH (57.24%), TSS (96.54%), DO (96.31%), BOD5 (100%), Turbidity (96.77%), and chloride (100%), fluoride (100%), phosphate (91.11%), COD (100%) and Copper (0.00%). The results showed that increasing coagulant dosage enhances water quality with respect to all parameters studied. Furthermore, the results indicated that pH, DO, BOD5, fluoride, phosphate and COD mean % efficiencies were higher for ferrous sulphate coagulant in comparison with alum. On the other hand, alum displayed a better coagulation efficiency than ferrous sulphate in the following parameters TSS, turbidity and chloride. The overall results of the coagulation studies applying increasing coagulant dosage revealed that coagulant efficiency is parameter dependent. The outcome of this work can be an important guide to water treatment operators.

Thesis Overview

1.0       Introduction

Water is a basic necessity of life. It is a limited resource. So, preserving the quality of water is important for the drinking water supply. Water quality can be compromised by the presence of harmful agents. One of the major problems of using surface water as source for drinking water is the high content of natural organic matter (NOM), which has an adverse effect on the aesthetic quality of water and may increase corrosion and biofilm growth in the water distribution systems1,2. Furthermore, when water is disinfected in the course of its treatment (purification), chemical disinfectants can react with NOM to form disinfection by-products (DBPs) like Trihalomethanes (THMs) and halo acetic Acids (HAA)3. DBPs are considered to be potentially carcinogenic3,4. In 1998, the US Environmental Protection Agency (USEPA) regulated the THM and HAA at a maximum allowable level of 80 and 60µg/l respectively for drinking water5. Hence, NOM has to be removed from drinking water more efficiently. In drinking water treatment, coagulation process is the most important stage for the maintenance of acceptable treated water quality and economic plant operation6. Chemical coagulants are added to water to facilitate bonding among particulates that are widely used to enhance the removal of colloidal particles. Coagulation is not only effective in precipitation of particles, but also it has an important objective of removing pathogens. Many researchers have applied coagulation to remove turbidity and NOM7.

Some of the well known and common coagulants used are aluminium sulphate, ferric chloride and ferrous sulphate for water treatment7. Determining the optimum coagulant dosage for a given raw water is a major problem. Jar test procedure with a six unit multiple stirrer system has been commonly used to determine the required concentration of coagulant dosage7,8,9,10,11. This is generally carried out periodically10,11. But a four litre sedimentation beaker with a stirring bar inside it, and mounted on magnetic stirrer is used for this research work to determine optimum coagulant dosage in some parameters.

Optimum coagulant dosage is the lowest coagulant dosage at which maximum coagulation efficiency is achieved. The following parameters tested for coagulation efficiency: pH, TSS, DO, BOD5, Turbidity, Chloride, Fluoride, Phosphate, COD and Copper. During the sedimentation beaker test the level of one factor is set differently while levels of other factors are held constant. By varying only one factor, the operator can see how changes to that factor will affect the treatment process results. The effectiveness of the coagulation process is highly dependent on optimum coagulant dosage, raw water pH, mixing time and sedimentation time. Proper coagulation is essential to produce satisfactory treated water qualities, to maintain the economic value of the plant operation and also for DBP control. Poor control will cause wastage of chemicals, low water qualities and failure in the sedimentation and filtration processes.

In addition, excessive coagulant dosage has been linked to several medical disorders such as Alzheimer’s disease12. When there is high turbidity in water the physical properties of water is affected. Due to the water quality problems and stricter regulations for drinking water quality, plant operators have to use the sedimentation beaker test to determine the required coagulant dosage at any time. Again, jar test has limitations in that it is expensive, and time-consuming10,12,13. Sedimentation beaker test is cheap, saves time and easy to operate. Furthermore, depending on the pH after the coagulant is added, two possible reactions are generally possible.

*With aluminium-based coagulants, the metal ion is hydrolysed to form aluminium hydroxide floc as well as hydrogen ions. The hydrogen ions will react with the alkalinity of the water and in the process, decrease the pH of the water as can be seen from Equation below: for alum.

Al2 (SO4)3. 16H2O→ 2Al3++3 +16H2O→ 2Al (OH)3 +6H++ + 10H2O –           1

Similarly for ferric sulphate, the following reaction takes place:

FeSO4.7H2O→Fe2++  + 7H2O→Fe(OH)2 +  + 2H+ + 5H2O            –           –           2

The above hydrolysis reactions typically take place at a dosed water pH in the range 5.8 to 7.5, depending on the particular coagulant. Colour and colloidal matter is removed by adsorption onto/within the metal hydroxide hydrolysis products that are formed, and is sometimes referred to as sweep-floc coagulation.

*If an excess coagulant is added so that the dosed water pH is less than 5.0, then the metal ions will directly neutralize the negatively charged organic compounds and colloids in the raw water. This allows the organic molecules to contribute to floc formation and is often referred to as enhanced coagulation and is often done to boost the removal of disinfection by-product precursors14.

            Furthermore, industrial waters are clarified to remove turbidity and color from the effluent streams in the textile, paper and other polluting industries15. The dictionary meaning of a coagulant is an agent that induces curdling or congealing. In a water treatment, what it is, a chemical that will remove color and turbidity present in raw water in the form of flocs. Coagulants neutralize the repulsive electrical charges (typically negative) surrounding particles allowing them to “stick together” creating clumps or flocks. Flocculants facilitate the agglomeration or aggregation of the coagulated particles to form larger floccules and thereby hasten gravitational settling. Some coagulants serve a dual purpose of both coagulation and flocculation in that they create large floc’s that readily settle.

In wastewater treatment, coagulation and flocculation are employed to separate suspended solids from water. Although the terms coagulation and flocculation are often used interchangeably, or the single term “flocculation” is used to describe both; they are in fact, two distinct processes. Knowing their differences can lead to a better understanding of the clarification and dewatering operations of wastewater treatment. Finely dispersed solids (colloids) suspended in wastewaters are stabilized by negative electric charges on their surfaces, causing them to repel each other. Since this prevents these charged particles from colliding to form larger masses, called flocs, they do not settle. To assist in removal of colloidal particles from suspension, chemical coagulation and flocculation are required. These processes, usually done in sequence, are a combination of physical and chemical procedures. Chemicals are mixed with wastewater to promote the aggregation of the suspended solids into particles large enough to settle or be removed.

Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. Cationic coagulants provide positive electric charges to reduce the negative charge (zeta potential) of the colloids. As a result, the particles collide to form larger particles (flocs). Coagulation thus implies formation of smaller compact aggregates. Rapid mixing is required to disperse the coagulant throughout the liquid. Care must be taken not to overdose the coagulants as this can cause a complete charge reversal and restabilize the colloid complex.

Effluents are heterogeneous in nature. Chemical coagulation is an important unit process in water treatment for the removal of turbidity. Its application in water treatment is followed by sedimentation and filtration. Various types of coagulants are being used to condition water before sedimentation and filtration15.

•     Statement of Problem

The following water related problems have inspired the researcher into this project.

• Some types of water in their natural forms contain micro-organisms, organic matter and mineral. Thus they require some treatments before consumption.
• People living in rural areas of the developing country like Nigeria find it difficult to have access to potable water and thus resort to drink turbid water due to lack of water treatment plants.
• The operators of the water treatment plant in Nigeria pay high price in search of expensive coagulants to use in water treatment rather than looking for cheap easily accessible coagulants.
• Human activities have contributed to poor quality of water in our environment which make the water unfit for domestic and industrial uses.
• In January 2015, the Enugu State Government announced the outbreak of cholera in Nsukka and its environs, this problem was attributed to poor quality of water that people in the area consume.

1.2       Aim and Objectives of the Study

The aim of this study is to compare the performance of two selected coagulants (alum and ferrous sulphate) in water treatment. Consequently, the following specific objectives are applicable.

• To determine coagulation efficiency of each coagulant with respect to each of selected water quality parameter set of parameters.
• To assess the coagulant dosage in relation to optimum coagulation efficiency.

1.3       Scope of the Study

In this study two coagulants, namely ferrous sulphate and alum, which are among the most common types of coagulants in water treatment plants in Nigeria, were investigated with the aim of determining their capabilities to reduce turbidity of drinking water. Various water quality parameters including: pH, Turbidity, Total suspended solids (TSS), Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD5) phosphorous, chloride, fluoride, copper and Dissolved Oxygen (DO) are determined to find out the removal efficiencies of the coagulants.

1.4       Justification of the Study

Considering general acute scarcity of natural good quality water, the importance of water treatment according to standard specifications cannot be over emphasized. A critical stage of the treatment process is coagulation where the choice of appropriate coagulant is very essential. This work was designed to highlight the comparative coagulation performances of two popular water treatment coagulants- alum and ferrous sulphate. From my literature survey, not much work has been done in this area in the past.