Introduction & Brief:

Potable water serving the greater Gort area in Co. Galway is supplied from various sources, which are of

inadequate quantity & quality. The local authority wishes to discontinue use of the existing supply sources

and replace these with a new single surface water supply source. Thus a Hydrology report was compiled

addressing the following:

- Identify new water sources - Suitable locations for abstraction on this source

- Hydrological and Hydraulic impacts of abstraction - an inflow series & impacts of abstraction

- General Hydrology, Annual W.B.E. and average Run-off of Catchment chosen

- Rating curve(s) for lake outflow - Are impoundment measures required? If so, what?

It also wishes to decommission the existing water treatment plant which is nearing the end of its useful

life and provide a new plant at the location indicated on the enclosed map. Thus a Water Engineering re-

port was compiled addressing the following:

- Identifying a suitable location for the new facility

- Preliminary design of the main treatment processes and calculations for each (e.g. Sedimentation,

Filtration, Disinfection, Sludge management) and a scaled layout of the proposed plant.

The following information was also supplied:

a) The Population Equivalent (PE) of Gort town & its environs is 52,500. A future allowance of 20%

should be included for design purposes;

b) A required flow of 220l/s is required at all times in the Gort River to provide adequate assimilative

capacity for Gort wastewater treatment plant.

Hydrology: source analysis

Sources: The possible sources identified are those labelled below, For various

reasons, Lough Cutra was the chosen solution, as below.

With the source chosen, and its Catchment Area calculated (Red outline), The

following simplified W.B.E. had to be applied:

This is a mathematical equation which is governed by the chosen constituents

of the Hydrological cycle (see fig. 2), as taken over a hydrological year.

The quantities which are taken into account are: Precipitation (P), Evapotranspi-

ration (E), Surface Run-off (Q) and Change in Storage (ΔS).

These terms are simplified and re-arranged to: This is due to the fact

that this W.B.E. is taken over a Hydrological year, thus ΔS ≈ 0.

The average Catchment run-off (Q) over a hydrological year is thus:

The abstraction rate (q) required for the Greater Gort area is:

River Turra

River Beagh

Lough Cutra

Gort town and sur-

rounding area

Hydrology & Water Engineering:

A new water source and treatment plant for the greater Gort area.

Catchment Area =

150.2m2

Q = 3.807m3/s

q = 0.116m3/s

Fig. 2: The Hydrological Cycle

Fig. 1: The Greater Gort

area. Co. Galway

Q = P - E

P = Q + E + ΔS

Hydrographs: The flow rate required (q) is exceeded 93% of the time according to

Fig. 3, however this was

deemed unacceptable as it

amounted to roughly 1 in 20

days, statistically, having an

insufficient flow rate. It was

decided we need the required

flow rate (q) 100% of the

time. The 100 percentile value

is 0.022m3/s, which is not

sufficient for our design.

Fig, 3 : Flow Dura-

tion curve

The Rating curves: (Fig 4 & 5) were used to find a relationship between Stage (the height h)

and the corresponding outflow (flow rate Q). As one can see from Fig. 5, There is three differ-

ent “stages” (labelled 1-3), as can be seen with a log-plot of the data. As the R2 value for the

3rd order polynomial trendline (Fig. 4) is approximately equal to 1, and the flow rate we’re

interested in is within stage 2) of the graph, fig. 4’s data (or more precisely, the trendline

equation) shall be used to find a relationship between flow rate and stage, and therefore

(with an area-height graph plot (from Fig. 7)), a stage-storage relationship was established.

Fig. 4: rating curve Fig. 5: linear rating curve

Fig. 7: Area-Height Fig. 8: Stage-Storage Graph

Hydrorouting: The principle of back-routing was applied to our Lake, so was to estimate the

inflows that would result from our calculated stage-storage relationship and the given outflow

data from EPA. This stated mathematically is: (It+Δt + It)/2 = (Qt+Δt + Qt)/2 + (St+Δt – St)/Δt

This resulted in the following plot for the inflow series (2013 data, taken at 3 hour intervals):

Forward routing should have been undertaken so as to confirm that the calcu-

lated inflow series (from backrouting) resulted in the same outflow series for the given data,

however this was not done in reality, but if time allowed, this should have been done so as to

test the validity of the inflow series calculated. The equation for forward routing is:

With a correct model for Hydrorouting created, it was simply a matter of adding our abstrac-

tion rate (q) to the equation above and resolving:

(Qt+Δt + Qt)/2 = (It+Δt + It)/2 – (St+Δt – St)/Δt

Qnew = 2*(I1+I2)/2 – q – (S1-S2)/Δt+ - Qo

The graph above shows a comparison between the behaviour

of the lake before and after abstraction.

Fig. 9—Routing graph

The graph above shows that, for the longest drought period (July-August ‘86), with

abstraction, The flow rate is not sufficient naturally, Impoundments may be needed.

Name: Conor Meaney

I.D.: 11138874

Course: 2nd year Civil Eng.

Module: CE4014

Project: Trigger 1 (a) & (b)

Date: 30/04/13

Water Engineering: Treatment Plant Design

Conclusions & Recommendations:

Plant design: 1) The plant is of insufficient size and thus more area may need to be

purchased for future extensions. 2) As the plant is of insufficient size and the raw wa-

ter storage on site is not critical, this area could be used instead for future extensions

or for onsite screening. 3) The plant should be mechanised as much as possible so as

to reduce labour costs and increase efficiency.

Lough Cutra: As there is insufficient flow rates at certain periods (mostly droughts,

etc), the installation of a weir as below will permently rise the lake level and allow

for pumping when insuffificent flows are outflowing, this is an economical solution.

The Plant Location: The location for the plant was already allocated (Fig. 10 & 11),

however the site was the most logical as it had many geographical and hydrological/

hydraulic advantages as it lay close to Gort and the Abstraction point, whilst also having

main road and minor road access to the site. There is also land available for further pur-

chases for expansion, amongst other reasons not highlighted here.

Fig. 10: Plant Location in Gort area Fig. 11: Site & Dimensions

Primary Treatment Process Design:

Screening: Upon entering the facility, the raw water will be screened twice, so as to pre-

vent damage to mechanical equipment by large and small objects alike. The first screen

will be a Coarse Bar (“trash bar”) screen (Fig. 12), of standard steel sections, e.g. 15mm

diameter and 50mm spacing’s, This will prevent large objects like branches from entering

the facility. The second will be a Fine screen with a mesh of 5mm length steel squares

(Fig. 13), so as to prevent leaves or other small objects from entering the facility. Both will

be mechanically cleaned via backwashing and the waste disposed of in a landfill or skip.

Raw Water Storage: This is the amount of raw, screened water which will be stored for a

certain period required, that period depending on the risk of contamination/pollution

upstream. As our catchment is relatively unspoilt and thus a low risk of contamination, a

retention time (Tr) of 24hours was chosen. The flow rate (q = 0.116m3/s) is that of the ab-

straction rate, including the 20% increase for design purposes. The volume required was

thus calculated from: Vtank = q x Tr = 10,023m3 (See Fig. 14)

Fig. 12: Coarse bar Screens Fig. 13: Objects in Fine mesh

Coagulation: This tank was sized according to the

above equation also, however Vtank was capped at

8m3, thus choosing a retention time of 1minute

(60s), the volume was calculated:

Vtank = 6.96m3

P = G2μwV and P120 = Ktn3DI

5ρw

Fig. 14: Raw Water Storage Tank

Fig. 15: Coagulation Tank

As this was mechanically mixed, it has to have

a mixer with a power supply governed by:

The Ratio of Di over D had to be within a

range and the flow structure had to be turbu-

lent for the design to work, as shown

in Fig. 15. Flocculation This tank is designed similarly to Co-

agulation

It is slowly mixed to promote agglomeration of

particulates and there is no cap on the volume.

The equations are the same as that for Coagula-

tion however the paddle speed must be within a

certain band governing by

(see Fig. 16): Fig 16. Flocculation Tank

Sedimentation/Settlement: This is governed by the following equation:

Fr should be as close to one as possible so as to reduce the

concentration of suspended solids to nearly zero. For our design purpose, the turbid-

ity had to be less than 5NTU after sedimentation.

Filtration: This is governed by the following equation:

Re-arranging and solving for z/d allowed us to calculate a ratio of media particle di-

ameter to media depth, and thus calculate the required depth of each media to re-

duce the turbidity to the required 0.3NTU or less. The filter must be backwashed reg-

ularly for efficiency and hygiene reasons.

Fr = (1 - fo) + (1/vo)∫vdf

Sludge Management: Sludge from the Sedimentation and Filtration tanks will be

sent here for processing via de-watering (in an oven) and thus into 90%wt solids.

Disinfection: Water leaving the Filtration tank will be disinfected via Chlorination

and UV Irradiation so as to remove all major pathogenic risks, while also allowing for

some residual disinfection in the supply by excessive chlorine.

Plant Layout, Scaled 1:200 (See below)