FIJI NATIONAL UNIVERSITY COLLEGE OF ENGINEERING SCIENCE AND TECHNOLOGY

Derrick Campus

BEC 710

Water Resource Engineering

Field Trip Report SEMESTER I, 2015

YEAR 4

Student : Deepak R Prasad

ID # : 2006001699

Abstract

This paper will explore the process of water and sewerage treatment for Suva City, Fiji Islands.

Firstly, an evaluation of the treatment plant visited will be discussed. Furthermore, future expansion

and vision of the Plant will be discussed. Finally, this report will conclude that conventional

treatment is very effective form of treatment.

Introduction

The total amount of water in the world has been estimated in categories of fresh, land-locked saline,

ice caps and glaciers and the oceans. From these categories, scientist estimate that 97.3 % of the

planet's water is in the oceans. Of the remaining 2.7 %, 77.2 % are trapped in the polar ice caps and

glaciers and land locked seas. While, another 0.3 % comprises of saline lakes. This leaves

approximately 22.5 % of the freshwater potentially available for use as water supply.

To produce drinking water, water is extracted from the underground, so called groundwater, or from

rivers or lakes, the surface water. This source water is not drinkable and must be treated. The

treatment plant consists of different processes depending on the quality of the source and the

requirements for drinking water. Afterwards, the water is transported to the city and distributed via

a piped network to households, commercial buildings, public entities and small industries. After

usage the water is collected in a sewer system and transported to a wastewater treatment plant

where the water quality is improved for discharge into the receiving water bodies. These surface

waters can then, again, be used as a source for drinking water production.

For the purpose of this report, investigation will be carried out to ascertain the nature and form of

Sewer & Water treatment done for Suva City. Furthermore, other modern options of treatment will

be discussed.

Methodology In order to learn more about water and sewerage treatment, several research methods were

employed.

Observation A site visit was carried out to observe the general layout of the treatment plant and acknowledge

the process adapted.

Interviews The following people were interviewed for their perspective.

WAF Staff – Mr. Vilitati

WAF Staff – Mr. Masi

Limitations The time frame was short. It is believed that the questions asked provided sufficient information.

Results

The findings for the two sites visited are as follows.

CASE 1 – Waila Treatment Plant (WTP)

The plant is operated by Water Authority of Fiji (WAF). It is located a bit further away from Nausori

Town, at an elevated Reduced Level for gravity supply to Nausori residence. Built in the 1980’s, the

plant follows typical conventional water treatment process with raw water sourced from nearby

Rewa River. Plant Capacity is 97ML. The processes adopted at WTP are as follows;

1. Screening

This is done at the intake. Screens are used to remove coarse and floating objects.

2. Coagulation

Is the addition of chemicals (Alum & soda Ash) into the water to artificially increase the settling rate

of fine suspended particles in Raw water. Alum (Aluminium Sulphate) is added to a water. The

chemical reaction is as follows;

The negative charge particles are responsible to supersede the Van-de-waal forces and bind the

suspended particles. In cases where pH is low, Soda Ash (Sodium Carbonate (Na2 CO3 )) is added as

control. Dosage is based on Jar test results.

Picture of WTP Soda Ash Batch tank

3. Flocculation

This is a slow mixing process in which destabilized colloids are brought into contact to promote

agglomeration. The objective of flocculation is to bring the particles into contact so that they will

collide, stick together, and grow to a size that will readily settle or filter out. WTP has a baffle wall

flocculator.

Picture of WTP baffle type flocculator

4. Sedimentation

At this stage impurities are removed. Substances that cause colour and turbidity and, substance that

have become settable by coagulation and flocculation settle and extracted out as sludge. WTP has

two types of Settlement tanks or Clarifiers

Circular Clarifiers

In this tank, the flow is introduced into a central well (small open ended cylinder) via a pipe below

water level but high enough off the floor to avoid the scrapers. The water flows outwards to the

outer walls and is collected at the surface. As it travels, the area of the flow increases and the speed

decreases. The particles settle out into the quieter water below and fall to the floor. Either scrapers

rotate slowly and sweep the floc into a central well or the bottom is hopper shaped to collect the

sludge.

Picture of WTP circular clarifier.

Tube and Plate Clarifier

The concept is putting several horizontal layers into the same tank to reduce the amount of land

required. This is achieved by "plate separators" wherein the simplest versions have multiple flat

plates mounted one above the other. These plates are inclined about 60º to the horizontal to make

the sludge flow down under it's own weight to a collection hopper. Here clean is removed without

re-suspending the solids. This is achieved by having the flow travel up the plates in the opposite

direction to the solids flow - the "counter-flow" type.

5. Filtration

WTP has 7 numbers of Rapid sand filters. In this stage of treatment, particular impurities and floc

from the water are removed and clear water is let to pass through. The apparent method by which

filters remove suspended material is straining - the particles are held back by having holes smaller

than the particles removed. However, secondary mechanisms are also important. These include

filter "ripening" - a buildup of larger particles on the surface of the filter that decreases the aperture

size allowing smaller particles to be removed. Another key secondary process is the settling of

particles onto the horizontal parts of the filter media. This settlement process is enhanced by the

curvature of the flow around the media, spinning the particles out to the edge of the flow through

the pores due to their higher density. There are also electrostatic forces which attract particles to

the media. After few days, the filters require backwashing when the pores become blocked with the

solids removed. In this instance Backwashing is carried out

Backwashing is initiated by any one or more of the following factors:

Time - wash filters on a regular schedule of say every 2 or 3 days.

Turbidity - if the turbidity from a filter starts to rise, this indicates that solids are being

pushed through the media and it needs to be backwashed - immediately.

Headloss - as the headloss rises, so does the probability of floc shear and particle

breakthrough. Reaching a pre-set headloss level will initiate a backwash.

Backwashing technique at WTP uses both air and water at the same time. The sequence starts with

air and water at concurrently. This wash lasts for about 5 - 10 minutes. It is followed by water alone

for about 5 minutes to remove any trapped air.

Picture of WTP Rapid Sand filter during backwash.

6. Disinfection

At WTP, chlorine gas is used. These are supplied large gas cylinders. Clorine dosage is depended also

on water pH. The higher the pH, the less effective the disinfection becomes. It is noted that going

from pH 7 to pH 8 halves the amount of disinfection. To counter this effect, the chlorine dose is

increased at higher pH levels or Soda Ash is used to reduce pH.

The chlorination system at WTP comprises of;

A chlorine drum source and a 12 mm gas line to the chlorinator.

A chlorinator.

A pipe from the chlorinator carrying the chlorine to the treated water main pipe where it is

injected.

The chlorinator controls the amount of chlorine gas drawn into the water. The control methods and

inputs include, the water flow rate and the residual chlorine level analyzed. Chlorine is added in the

clear tank.

7. Fluoridation

The fluoride injector is used to add the chemical. This added to prevent tooth decay. Fluoride is also

added in the clear tank.

8. ph Control

Soda Ash is added to control pH. Soda ash is the common name for sodium carbonate and this is also

known as washing soda. It is added to drinking water to control the pH values to between 7 to 8.5 -

as set out in the Drinking Water Standards.

Also, to limit the corrosion costs and aesthetic disadvantages of corrosion, the water pH is adjusted

to fall within the range set in the standards.

9. Flow Distribution

The treated water is stored in a 6ML reservoir. It feeds the Raralevu Reservior via gravity and

Wainibuku Reservior via high lift pumps. Both the tanks, then supply to the reticulation systems of

Nausori and Suva area respectively.

The minimum service pressure usually specified is 30 m of water head. The normal maximum is

120m. Pressures higher than this may be supplied with resulting risks of fitting failures, excessive

wear and tear on tap washers and automatic valves such as on dishwashers or automatic washing

machines. Many plumbing fittings are not rated to higher pressures - items such as pressure

reducing valves on gas califonts are prone to failure.

Schematic Waila Water Treatment Plant Diagram

CASE 2 – Kinoya Sewerage Treatment Plant (KSTP)

This plant is also operated by Water Authority of Fiji (WAF). It is located in between Suva City and

Nasinu Town, at a very low Reduced Level for gravity intake. Built in the 1970’s, the plant follows

typical conventional sewer treatment process. The Plant has the capacity to cater for a population of

180,000 persons. Storm water is not collected. The plant capacity is approximately 30ML/day and

has the ability to collect 250 – 800mg/L BOD sludge and treat it upto a reduced 30mg/L. The

processes adopted at WTP are as follows;

Source: Google Images (edited)

1. Screening

The Plant employs both bar followed by continuous screen as depicted in picture below. These are

used to remove any floating debris such as plastics that will affect other treatment process.

Bar screen Continuous screen Source: google images

2. Grit Removal

In the stage, sand and grit must is removed from wastewater. This is critical;

• to extend the lifespan of the mechanical components, especially pumps;

• to prevent sand and grit from getting into the pipelines and machinery, which can cause blockages.

• to avoid depositing a sand package at the bottom of the digestion tank, the presence of which

would minimize the effective volume and hence the efficiency of the tank.

3. Primary Treatment

The primary sedimentation tank comes after the grit chamber. Here as many of the settleable un-

dissolved particles as possible are separated. This sludge is called primary sludge. This reduces the

load and size of sludge treatment units. Almost 60% of all suspended solids are removed at this

stage.

4. Secondary Treatment

The trickling filter bed consists of a cylindrical tank 2 – 4 meters high equipped with a perforated

bottom. The trickling filter is filled almost entirely with packing material made up of lava slag, gravel

or other suitable material. A rotary distributor arm spreads the influent wastewater over the upper

layer of the media and then the wastewater trickles down over and in between the packing material

through the filter. Through the under-drain and the drainage collection trench the treated water is

discharged.

The stabilization of wastewater is accomplished biologically using a variety of microorganisms. The

so-called biological skin, also known as “biofilm” is developed on the surface of the packing material.

The settled wastewater flows in thin layers over the aerobic layer of the biofilm. From the passing

liquid absorption of the dissolved solids, present in the settled wastewater, takes place in the

biofilm. All of the biological contaminants are consumed and degraded, these lead to the treatment

of the wastewater.

5. Thickener

The stabilized effluent is passed onto this tank for thickening. The supernatant water is discharge

into the sea, 1.5 km from the shoreline. While, bottom thick layer laid to dry. The partially dry sludge

is belt pressed and formed into Cakes. It is sold as manure.

6. Sludge Digester

The plant also has a digester tank, where anaerobic metabolism takes place and Methane gas is

produce. This, WAF simply burnt for carbon credit.

Picture of Kinoya Sewerage Treatment Plant.

General Layout of Kinoya Treatment Palnt

Discussion

It is noticed that the method of treatment has remained same for the two treatment plant over the

years. However new electronic devices; such as pumps, chlorinators, motors, control meters, and

the likes have drastically improved the plant efficiency and ability to cater for the demands of the

growing population.

For the water treatment, WAF is looking at avenues to find quick treatment solution. Other methods

of treatment are being looked at to supply water to evacuation centers during storm periods. AVG’s

and package plants seem to be cost effective solutions.

Sewerage on the other hand, is still an area of research. Recently a break thru findings was unveiled.

Nereda process in the Netherlands, claim that sewerage plant capacity could be increase just by

using different bacteria. This new found bacterium reduces the tank detention time and thus,

increases plant efficiency. There are also studies carried out to turn Sewerage Treatment Plant into

an energy producing plant for methane gas.

Conclusion

The conventional method of treatment is still very effective. The process employed is direct and

staged. As such, confusion or complications does not arise when identifying faults. For example,

smell in sewer treatment plant or high pH level in water supply. By simply adding soda Ash or

correcting the screen chamber the problem could be solve respectively. All in all, the plant are easy

to maintain and reliable for long term use.

Bibliography

Metcalf and Eddy, 2003. ‘Wastewater Engineering Treatment and Reuse’, MacGraw hill, New Dehli.