Conceptual Storage and Discharge Option Assessment … · Conceptual Storage and Discharge Option Assessment At Daleton Farm, Carterton District Council ... performance of batch stabilisation

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Conceptual Storage and Discharge Option Assessment

At Daleton Farm, Carterton District Council

Version 9

Sustainable development, energy,

and environmental consultants

Storage and discharge options April 2017 V9

This report is copyright to EQOnz ltd ©2016 and may not be reproduced without consent. Email: [email protected] Tel: 06 3708175

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Contents 1. Executive summary ..................................................................................................................... 3

2. Introduction ................................................................................................................................ 3

3. Data used in this report .............................................................................................................. 4

4. The big picture ............................................................................................................................ 4

5. Batch Reservoir Basics ................................................................................................................ 6

6. Overseas experience ................................................................................................................... 8

6.1. General .................................................................................................................................... 8

6.2. Performance Criteria ............................................................................................................... 8

6.3. Odour issues ............................................................................................................................ 9

6.4. Shock loadings ......................................................................................................................... 9

7. Batch reservoir trial results ......................................................................................................... 9

7.1. Trial 1. ................................................................................................................................... 11

7.2. Trial 2 .................................................................................................................................... 12

7.3. Trial 3 .................................................................................................................................... 14

7.4. Conclusions from trials.......................................................................................................... 17

8. Buffer storage investigation ...................................................................................................... 17

8.1. The concept of buffer storage .............................................................................................. 17

8.2. Wastewater flow data ........................................................................................................... 17

8.3. Receiving water flow data ..................................................................................................... 17

8.4. Building a flow model ........................................................................................................... 19

9. Model assumptions & limitations ............................................................................................. 21

10. Receiving water sensitivity .................................................................................................... 22

11. Balancing wastewater quality against the discharge regime ............................................... 23

12. Preferred Option ................................................................................................................... 24

Works Cited ........................................................................................................................................... 25

mailto:[email protected]



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1. Executive summary

This document is intended to provide information to inform the assessment of the ecological effects

of the Carterton District Council 2017 consent application preferred option. It summarises

investigations undertaken regarding storage options in terms of effect on discharge regime and

effluent quality. It is part of, and should be read in conjunction with, the other 2017 consent

application documents.

Whilst sequential batch reservoirs offer additional treatment capability with protracted retention

times, greater instream environmental benefits are achieved by utilising the storage capability such

that discharges to water occur only at times of greater than 3x median flows. A balanced storage and

discharge regime is proposed such that a minimum rest time of 14 days is adopted (zero percentage

fresh effluent for 14 days – mean residence time will be much greater) in conjunction with

discharges to water at over 3x median, and land irrigation via centre pivot.

A daily time step analysis of daily river and wastewater inflow over the 2009-2015 period indicates

that discharges to water can be limited to times of only 3x median flow or greater for the vast

majority of conditions if 200,000m³ of storage is provided, and no less than 2x median flows for all

but extreme events. The modelling of this scenario indicates significant reductions in number of days

of discharge to water and total volume of discharge to water with good safety margins under most

conditions. Nevertheless it would be prudent to allow some flexibility in consenting to allow for

exceptional or unforeseen circumstances.

The optimisation of retention time is a matter for adaptive management; the balancing of

management of fewest days of discharge against risk of discharges in a ponds-full situation is as

much a community decision as an engineering one.

2. Introduction

As part of Council’s long-term strategy to avoid, minimise, and mitigate effects of the township’s

wastewater discharge on the Mangatarere Stream, Council instigated a research programme

investigating the potential to use a Sequential Batch Reservoir system (SBRes) to improve

wastewater quality and provide buffer storage.

SBRes have been used extensively overseas in countries like Israel where summer water irrigation

demand can only be met by recycling wastewater (Juanico, Wastewater reservoirs, 2005). These

reservoirs have been shown to perform treatment processes on the wastewater (Juanico, The

performance of batch stabilisation reservoirs for wastewater treatment storage and reuse in Israel,

1996), which whilst a by-product in terms of Israel’s irrigation to land could be significant in terms of

a discharge to water in New Zealand.




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A series of small scale trials were undertaken to estimate the performance of batch reservoirs in

New Zealand conditions.

Council concurrently commissioned a study by Tonkin & Taylor into the feasibility of constructing a

storage reservoir at the Daleton Farm site (Annex 1).

Also during this period, Council undertook liaison meetings with Greater Wellington Regional Council

to gather collective knowledge on options to reduce the overall effects of Carterton’s wastewater

scheme.

This document describes the investigations undertaken and the pathway leading to the preferred

option in terms of discharge regime. It does not attempt to quantify the effects of the discharge,

which is dealt with elsewhere.

3. Data used in this report

Table 1 Data sources

Type Location Unit Frequency Source Period

Flow Mangatarere SH2 m³/s 15min Greater Wellington Regional Council

2009-2015

Flow Wastewater treatment plant inlet

m³/s daily Carterton District Council

2009-2015

Rainfall Wastewater treatment plant

mm daily Carterton District Council

2006-2015

Evaporation Taratahi/Masterton aero

mm monthly NIWA

4. The big picture

This work forms one aspect of the larger project. Figure 1 indicates how different aspects are

related.




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Wastewater inflow, irrigation

rates, land irrigation area,

storage volume.

Input criteria governing

discharge regime

Discharge regime

River flow regime

Reservoir storage retention

time

Estimate effluent quality

Effects on receiving water

Daleton Farm land

optimisation strategy

Figure 1 Theoretical approach for evaluating effects




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5. Batch Reservoir Basics

Sequential batch reservoirs operate by sequentially filling and emptying a series of reservoirs (fig. 2).

By doing so, there is in intrinsic ‘rest’ period where wastewater remains in a steady state with

neither inflow nor outflow. In terms of contaminant concentrations, this is an important point. For

example, in batch reservoirs e-coli concentrations typically die-off at around 50% per day when the

incoming food source is removed.

Different contaminants have different amelioration rates and optimum conditions, which is further

complicated by the fact that each daily inflow has a different age from the previous or future day. In

order to estimate effluent quality, it is necessary to undertake a complex computer analysis that first

predicts the age distribution of the effluent.

Figure 2 Basic conceptual sequential reservoir flows

To do this a process diagram is required to model the potential effects of any component that

influences the daily flow regime and hence retention time (fig.3). This process diagram informs both

a flow/discharge model, and a Finite State Machine (FSM) software analysis for calculating

percentage fresh effluent and age classes of water in the reservoirs.




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Figure 3 Sequential batch reservoir process diagram




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Figure 4 Process diagram for Carterton Sequential Batch Reservoirs

From this process diagram, the reservoir fill state can be estimated (fig. 5), along with the

percentage fresh effluent (a key determinant of water quality) for any given age group.

Figure 5 Hypothetical example of reservoir fill state for a given scenario

The intent of this modelling is to combine water age classes with kinetic rate constants to estimate

the overall effluent quality at any time of discharge so that the environmental impact of a discharge

to water can be estimated. The FSM modelling will be used as an iterative management tool to

optimise storage protocols, but needs feedback from outlet water quality before it can be finalised.

6. Overseas experience

6.1. General Sequential batch reservoirs evolved in Israel, but have been used or trialled in Canada, USA, Brazil,

Chile, Italy, Germany, Morocco, China, India, and Spain. The primary drivers for their use are typically

an irrigation need or need to avoid discharge to water in critical times (Juanico M. , 1999). Significant

work has been done to understand the operational mechanics of these ponds, and the basic design

criteria established overseas has been used in the Carterton District Council process to modify

typical New Zealand dam design practice.

6.2. Performance Criteria Extensive and detailed modelling is carried out overseas to estimate the performance of batch

reservoirs, however without experimental evidence from New Zealand, it is not possible to

accurately predict the effluent water quality, as removal mechanisms for various contaminants are




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complex and related to meteorological criteria. Nonetheless discussion with arguably the leading

world authority on batch reservoirs suggests that there would be no particular obstacles because of

the climate1. Carterton District Council has trialled small scale holding tanks and gained sufficient

information to be confident that performance will be similar to overseas experience.

6.3. Odour issues The function of non-steady state reservoirs cannot be directly compared to that of steady state

ponds such as oxidation ponds, where the inflow basically equals the outflow. Overseas experience

shows that optimal treatment occurs when influent loading rates are less than 40-50kg/Ha/d

Biological Oxygen Demand, and that this is effectively the rule of thumb design loading rate to avoid

odour. However there are other factors that come into play – for example 100g/m³ of BOD loading

from an oxidation pond imposes less oxygen demand than 100g/m³ coming from an activated sludge

plant.

Odour is more likely to occur in deep, static reservoirs, and conversely higher loading rates can be

sustained in shallower reservoirs, or by the addition of aeration or mixers. The reservoir proposed by

Carterton District Council have a surface loading rate at the lower end of the design spectrum, are

relatively shallow compared to overseas examples, and have the ability to have aeration added at

any time. It is likely that minor aeration will be added to inhibit stratification anyway, but more

intensive subsurface or surface aeration is readily achievable. Odour is therefore not considered to

be a significant risk. Certainly no odour was detected from the trials (even under high loading), and

odour issues have not occurred with Pond 2 of the current wastewater treatment plant.

6.4. Shock loadings SBR’s can receive sporadic shock loads without adversely affecting the operation of the system as

the relatively large volumes offer a buffering capacity (Shilton, 2005). In addition this buffering

capability can effectively remove peak loading rates when compared to direct discharge systems, in

effect tending the maximum instantaneous load towards the mean.

7. Batch reservoir trial results Three trials were undertaken, one in 2015 and two in 2016. The first trial utilised the recycled water

reservoir at the wastewater treatment plant (fig. 6), whilst the two in 2016 were carried out in

specially constructed concrete tanks some 3m deep (fig 7).

1 Email correspondence with Marcelo Juanico




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Figure 6 Recycled water reservoir

Figure 7 Sample sat on reservoir trial tank




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We are neither aware of any Sequential Batch Reservoirs in Australasia, nor any trials. The trial

methodology was therefore compiled from a basic understanding of the processes likely to occur,

tempered by overseas data on reservoir performance.

The purpose of these trials was to increase understanding of the way in which these reservoirs

would operate in temperate climates, and to assess the likely treatment performance under New

Zealand conditions. This fed rate constants into the batch reservoir modelling to predict effluent

quality.

Figure 8 Internal processes in batch reservoirs (Friedler.E, 2003)

7.1. Trial 1. Trial 1 was carried out by filling the recycled water reservoir with final effluent from the wastewater

treatment plant – taken from the post-wetland pump chamber. Samples were consistently taken

from 500mm below surface level. The water level was not altered over the trial period other than by

natural evaporation and rainfall. The first trial had some limitations identified during the course of

the trial:

The relatively large surface area of the pond compared to the base area had the potential to

skew results from rainwater diluting the stored water.

A leak was identified during the course of the trial.

Nonetheless, the trial was useful in identifying overall trends, and after 26 days, recorded

contaminant reductions were as follows:




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Table 2 Trial1 contaminant reductions

Trial 1 , recycled water reservoir June/July 2015, 26 days

Contaminant Approximate reduction at end of trial

Suspended solids 50%

Biological Oxygen Demand 60%

E-coli 95%

Minor reductions of nutrients were also noted, however the trial inaccuracies made the findings in

this regard unreliable.

7.2. Trial 2 Trial 2 commenced in February 2016 and continued for 56 days. The trial was carried out in two

concrete open-topped vessels approximately 3m in diameter and 3m tall. Both tanks were filled with

final effluent from the wastewater treatment plant – taken from the post-wetland pump chamber.

Sampling commenced on the day of filling, with daily e-coli sampling and weekly sampling of other

contaminants.

Samples were consistently taken from 500mm below surface level. The water level was not altered

over the trial period other than by natural evaporation and rainfall. Average temperature was

around 20°C.

E-coli measurements were curtailed after ten days as concentrations had dropped to detection

levels (fig. 9). Recorded levels were comparable to overseas results where 50% die-off per day is

noted.

Figure 9 E-coli die off Vs time and theoretical die-off

Laboratory testing showed variable results depending on the contaminant type, with consistent

changes in pH, Total Nitrogen, Ammoniacal Nitrogen, Free Ammonia, and Nitrate/Nitrite (fig. 10).

Phosphorus reductions were 2% in one tank and 20% in the other. Both tanks suffered from what

appeared to be either wind or thermally generated re-suspension of solids, causing spikes in the

Total suspended solids and Biological Oxygen Demand readings. Following this dissolved Biological

Oxygen Demand and Dissolved reactive phosphorus were added to the future testing to investigate

the influence of particulates on Biological Oxygen Demand and total Phosphorus. It seems most

likely given the time of year (autumn with falling temperatures), that the material settled during the




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first part of the trial was disturbed by temperature inversion as temperatures decreased during the

latter stages of the trial (as is typical of oxidation ponds)2.

The key learning from this trial was the effect of batch reservoir storage on the Nitrogen cycle (fig.

10). Whilst dependent on temperature, the batch reservoir trial showed that both nitrification and

denitrification can occur without any external influence.

Table 3 Trial 2 results

2 https://www3.epa.gov/npdes/pubs/faclagon.pdf

Date

pH TSS TN NH3 NH4 NO3+NO2 TKN TP BOD filtered BOD

17/02/16 7.8 3 32.9 23.8 10.1 18

25/02/16 8.2 17 27 1.5 22 0.034 27 9.4 9

03/03/16 8.6 12 22 2.8 18.5 0.67 22 9.3 9

10/03/16 8.3 81 26 1.1 12 5.3 20 9.7 19

16/03/16 8 32 21 0.32 7.8 7.4 13.9 9.7 10

29/03/16 8.4 31 13.7 0.01 0.02 7.7 6 9.8 16

06/04/16 8.6 89 14.3 0.018 0.112 3.6 10.8 10.2 24

13/04/16 9.2 137 14.1 0.09 0.195 1.98 12.1 9.9 25 11

reduction -17.9% -4466.7% 57.1% 94.0% 99.2% -5723.5% 55.2% 2.0% -38.9%

Date

pH TSS TN NH3 NH4 NO3+NO2 TKN TP BOD

17/02/16 7.81 55 33.4 23.4 10.2 26

25/02/16 8.2 17 26 1.43 22 0.037 26 9.5 11

03/03/16 8.4 112 22 1.9 17.6 0.69 21 9.1 7

10/03/16 8.4 116 29 1.38 12.5 4 25 10.3 15

16/03/16 8 25 20 0.36 8.7 6.5 13.9 9.4 11

29/03/16 7.8 19 14.9 0.012 0.36 8.7 6.1 9.7 12

06/04/16 8.6 4.6 7

13/04/16 9.1 51 9.9 0.035 0.08 4 5.8 8.2 12 2

reduction -16.5% 7.3% 70.4% 97.6% 99.7% -10710.8% 77.7% 19.6% 53.8%

SBRes 2

SBRes 1


https://www3.epa.gov/npdes/pubs/faclagon.pdf



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Figure 10 Results of Nitrogen species

As can be seen almost complete nitrification occurred and partial denitrification resulting in an

overall 60-70% reduction in Total Nitrogen. The quiescent conditions favour settlement of dead

algae, suggesting that a) sludge production will be a matter for consideration and b) that there may

be stratification occurring with aerobic conditions at or near the surface favourable for nitrification,

and anaerobic conditions near the bottom favourable for denitrification.

7.3. Trial 3 Trial 3 was aimed at investigating a) the effect of lower temperatures on the nitrification process,

and b) the effect of aeration (note tank SBRes 2) (fig. 11).

In this trial effluent was taken direct from the tertiary pond (pond 2). It appeared that the effluent

quality from pond was less stabilised than the effluent taken post-wetlands, with some spikes in the

e-coli readings, possibly because of more particulates shielding and providing nourishment for

pathogens. Nonetheless e-coli levels reduced to detection levels within 16 days.

Overall the trial provided good base data for

estimating treatment performance relative to

overseas data. Trial 3 is of particular importance

given that it covered the season where the

reservoirs are more likely to be discharging to water,

and also the season likely to have the lowest rate

constants as many biological processes are

temperature related.

Significant differences were noted between the

operation and performance of the aerated and non-

aerated tanks.




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Figure 11 Air diffuser set up in Trial 3, tank 2

The aerated tank showed complete

ammoniacal Nitrogen removal from

nitrification and more consistent

suspended solids concentrations, which

is thought to influence other particulate

contaminants (like total Phosphorus).

The dissolved Biological Oxygen Demand

concentrations dropped by around 95%

for both aerated and non-aerated tanks,

the majority of which occurred in the

first two weeks (fig.12).

Figure 12 Dissolved Biological Oxygen Demand - Trial 3

Note that the results below are conservative in terms of Ammonical Nitrogen, Biological Oxygen

Demand and e-coli as all flow in real life passes through the existing wetlands, reducing

contaminants levels, and under the current scheme, also through the UV plant.

Mean Biological Oxygen Demand levels for the actual reservoir inflow load is expected to be around

32g/m³.




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Figure 13 Summary of results from Trial 3




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7.4. Conclusions from trials The trials indicate that Sequential Batch Reservoirs have potential to complement other treatment

processes and that subject to scaling issues, treatment mechanisms appear to replicate the findings

of established overseas installations.

The treatment achieved varies in relation to time, temperature, and the potential to add aeration.

Treatment performances can be divided into two types, the first having relatively rapid exponential

type concentration reductions, and the second more linear, longer-term reduction patterns.

Thus relatively rapid reductions can be expected in e-coli, Biological Oxygen Demand, and suspended

solids, with more gradual or longer term reductions in Total Nitrogen or Phosphorus. There appears

to be a lead in time for the nitrification process that is temperature dependent and altered by

oxygenation. In terms of optimising discharges, retaining a minimum retention time of 14 days

would yield significant reductions in Biological Oxygen Demand, E-Coli, and Suspended solids. During

the summer months, when storage times will be longer, reduction in ammonia nitrogen and total

nitrogen can be expected.

One aspect that was not able to be tested was the incremental filling of the ponds. Until the ponds

are full, there are elements of wastewater of multiple age classes; hence each daily input will have

received a different amount of treatment on any given day.

Modelling is underway to estimate the fractional age classes and, using the trial results, the

corresponding overall water quality for any given input regime.

8. Buffer storage investigation

8.1. The concept of buffer storage Flows into the wastewater treatment plant vary both diurnally and seasonally, and may be in phase

or out of phase with the hydrograph of the receiving water. The ability to buffer flows is therefore of

significance in reducing impacts by ensuring that high wastewater discharges do not occur at times

of low flow in the receiving water.

The existing wastewater treatment plant has some capacity for live storage, but further storage was

identified as being beneficial in terms of mitigating the effects of a wastewater discharge. Following

the Sequential Batch Reservoir trials further investigation was carried out to estimate the ability of

the reservoirs to additionally perform a buffering role.

8.2. Wastewater flow data In order to analyse the buffering capacity daily wastewater inflows to the wastewater treatment

plant were estimated from flow records for the period 2009-2015. The data represents the best

estimate of the actual flows, and data inaccuracies or gaps were estimated where required.

8.3. Receiving water flow data Flows in the Mangatarere have been monitored for several years. The bulk of the data relates to

flows at the gorge, however the correlation between flows at the gorge and the wastewater

treatment plant are not as strong as those between the wastewater treatment plant and flows at

State Highway 2, where flows have been accurately recorded since 2009.Daily and 15-minute flows

from SH2 were retrieved from Greater Wellington Regional Council and used for this analysis. Key

flow attributes are indicated in figure 14.




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Figure 14 Key flow attributes from SH2 recorder measurements since 2009.




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8.4. Building a flow model In order to establish the effects of any discharge, it is necessary to know the volume and timing of

any discharge relative to the receiving water. This is modified by the capacity and operation

mechanism of the buffer storage provided.

For Daleton Farm, a daily time step model was created to predict the discharge regime and hence

the effects of the proposed wastewater treatment plant. The flow model includes influent

wastewater flow, existing centre pivot irrigation, proposed centre pivot irrigation, storage (including

evaporation and rainfall), and discharge to water.

Currently the wastewater treatment plant operates a discharge to land for December to mid-May,

and a discharge to water for the remainder of the year. Deficit irrigation is used such that field

capacity is never reached during the irrigation season.

Analysis of monitoring soil moisture data indicates that, at least in some years, irrigation will be

possible and beneficial in November, as both soil moisture and temperature would support plant

growth (fig. 14) and are within consent requirements.

Figure 15 Soil moisture and temperature November 2015

The modelled irrigation therefore includes a low irrigation rate for November.

Data was taken from actual river flows and wastewater inflows. The spreadsheet model uses

parameters that can be set to govern the timing and rate of discharge to water with a resulting

required storage capacity to ensure that these conditions are met.

A daily calculation estimates the storage that would have been required on that day. Allowing for

discharges at any time of the year when flows are over 3x median gives a required storage of around

250,000m³ (fig. 16). Note the Greater Wellington Regional Council data records are missing for the

period May-August 2012.




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There are frequent situations where river flows settle at just under 3x median, and subsequently for

discharges at over 2x median river flow, significantly less storage is required. Thus a target of 3x

median flow and minimum 2x median flow appears feasible (fig. 16). Alternatively irrigation rates

could be temporarily increased – so for example the 2014 November storage peak would be reduced

below 200,000m³ if irrigation was increased to 15mm/d for the month of November. The best

mitigation option to take will depend on the circumstances at the time.

Figure 16 Storage required discharging at only times of over 3x median (dotted blue) and 2x median (solid red)

At a 30:1 dilution in the receiving water, peak flow discharges of 4,188 l/s (maximum daily average),

or 7,807 l/s (maximum daily instantaneous) would theoretically be possible. In reality the discharge

will be limited by the capacity of the transport pipes, for which a peak flow of 600 l/s has been

proposed (51,840m³/d).Higher discharge rates are therefore feasible, although there are decreasing

returns in terms of piping and pumping costs relative to the amount of water able to be discharged

because of the relatively brief duration of the higher flows.

The exact performance is dependent on a number of uncontrollable criteria, and cannot be

guaranteed to meet every combination of rain/inflow/river flow combination. Nonetheless, the

proposed storage provides a significant reduction in both the number of days on which a discharge

to water would occur, and the effects of those discharges. The risk factors associated with

discharges outside this regime have been assessed and are detailed elsewhere in the assessment of

environmental effects.

Proposed storage capacity




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Full-day discharges and unlimited storage would lead to 16 days of discharge per year. In practice,

there is a continuous risk management exercise required to balance the benefit of reduced number

of days of discharge against the risk of reaching storage capacity and having to discharge at times of

less than 3x median river flows. With non-full-day discharges (i.e. using every opportunity to

discharge at any time the river flow is over 3x median) and 200,000m³ storage, there would be an

average of 85 days discharge a year. It therefore seems reasonable to aim for a regime whereby

discharges are restricted to 30-45 days per year. This could be progressively reduced by further (off-

site) storage and irrigation areas, increased irrigation rates, or reduction in yearly flow from

reduction in infiltration and/or water demand management.

9. Model assumptions & limitations There are several variables associated with modelling the discharge, and an inherent risk that some

combination of variables will differ from the conditions assumed.

The assumptions used are:

Wastewater inflows are taken from Carterton District Council records. There are periods

where data inconsistencies occurred, and for those periods the Council’s best estimate of

inflows has been used

River flows are taken from Greater Wellington Regional Council data at State Highway 2.

Data is limited to 2009 onwards, and has some missing data notably May-August 2012. Data

from the gorge monitoring site is considered too disparate in terms of location, magnitude,

and timing to use here

Evaporation monitoring was only installed at the wastewater treatment plant in 2014/2015,

so average monthly evaporation rates were taken from local NIWA stations and assigned as

a daily rate

Daily rainfall data from the wastewater treatment plant was used

Acceptable irrigation rates vary from year to year, and there is no method of predicting what

rates would be applicable on any given day. Daily rates are based onrecorded measurements

from the Carterton wastewater treatment plant. Irrigation rates have been conservatively

modelled on standard pasture crop evapotranspiration coefficient (Kc) and the lowest

evapotranspiration season recorded in the last 20 years.

A maximum discharge rate has been based on practicable pipe conveyance capability.

Whilst it is not possible to predict future flow patterns, there are conclusions that can be made

about the likely performance:

I/I reduction works can improve control over the discharge regime and reduce the impact of

peak flows on the efficacy of the wastewater treatment plant.

Typically, more sustained rainfall events lead to higher wastewater flow but also higher river

flows; droughts lead to less opportunities to discharge but corresponding greater irrigation

potential and lower inflows. Climate change effects are likely to lead to peakier river flows,

which favour high-flow discharge regimes

The existing discharge regime is not reported as having significant adverse effects on in-

stream ecology over the summer months. Continuation of the dominant summer land

irrigation and restriction of riverine discharges to periods of high flow will further reduce the

effects. There is therefore some leeway for adaptive management techniques to maximise




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this improvement without the temptation to try and extract the exact detail in advance from

an unsolvable equation.

10. Receiving water sensitivity It has long been recognised that the effects of a wastewater discharge increase as flows in the

receiving water decrease and vice versa. In discussions with Greater Wellington Regional Council it

became clearer that when flows in the receiving water exceed 3x median, it is deemed that typical

wastewater discharges have minor or less than minor effects on the receiving water because a)

typically the water quality is already very low, and b) because typically the flow has sufficient

velocity to active the substrate – meaning periphyton is dislodged and sedimentation of any nutrient

laden sediments does not occur. Hence both the immediate and longer term effects are mitigated or

avoided.

There are no water quality limits set in the Proposed Natural Resources Plan when receiving water

flows exceed 3x median. In terms of effects, it is therefore preferable to weight discharges in favour

of times of high river flow.

Analysis of flow data indicates the frequency that 3x median flows occur in the catchment (table 4).

Table 4 Distribution of 3x median flow occurrences 2009-2015

Excepting February, 3x median flows can be expected at any time of year, but the majority of events

occur May to October. Similarly from figure 14 it can be seen that with virtually no flows below half

median from June to October, this is the period of least sensitivity in terms of the receiving water.

The average number of days of river flow over 3x median is around 50, with the minimum being 34 if

the missing data in 2012 is taken into account. Analysis of the flow data reveals that the average

flow on days over 3x median is 16.78m³/s, significantly higher than 3x median (6.87m³/s). At a

dilution rate of 30:1 and a historical minimum of 34 days per year above 3x median flow, this yields

a minimum potential discharge capacity of 1.6Mm³ per year, roughly double the target wastewater

inflow. At the historical average of 50 days per year over 3x median stream flow, the average

potential discharge capacity is 2.4Mm³ - roughly 3x the actual average yearly wastewater flow. As a

2009 2010 2011 2012 2013 2014 2015 Min. mean Max

Jan 2 3 6 0 2 0 0 2.166667 6

Feb 0 0 0 0 0 0 0 0 0

Mar 0 0 6 1 1 0 0 1.333333 6

Apr 0 6 0 1 13 3 0 3.833333 13

May 7 5 4 7 2 2 5 7

Jun 20 0 11 5 8 0 8.8 20

Jul 4 16 6 6 0 0 6.4 16

Aug 7 6 5 5 10 5 6.6 10

Sep 4 26 0 8 10 4 10 0 8.857143 26

Oct 10 2 1 5 11 3 1 1 4.714286 11

Nov 0 0 2 0 7 0 0 0 1.285714 7

Dec 2 0 1 0 2 0 0 0 0.714286 2

Total 68 40 25 58 46 34 49.70476

Days of average flow> 3*median




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high level analysis it can therefore be seen that with sufficient storage capacity, there is ample

theoretical opportunity to discharge only at times of 3x median or higher river flow.

The ability of the system to meet the storage demand is therefore dependent on the timing

(frequency) of discharge at flows over 3x median.

Given the climate change predictions for the Wairarapa (greater frequency of high intensity rainfall

events, greater frequency of droughts) and the already peaky nature of the Mangatarere hydrograph

(fig 17), the avoidance of discharges at lower flows is likely to become more important. Similarly the

ability to discharge more at times of high flow is likely to reduce the overall effects.

Figure 17 Flow records SH2 for the Mangatarere

The proposed 2nd centre pivot irrigator gives greater ability to discharge to land, and given the

performance of the existing pivot it is fairly certain that apart from extreme and unusual weather

conditions, the land irrigation system will cope with summer and autumn wastewater flows if

discharges are allowed at flows over 3x median.

11. Balancing wastewater quality against the discharge regime At around 200,000m³, the capacity of the proposed reservoir provides an additional option of

buffering flows to optimise discharges based on receiving water flow rather than effluent quality.

Modelling real wastewater input flows and receiving water flows at State Highway 2 in the

Mangatarere, it can be seen that for the vast majority of the period modelled (2009-2015) it would

have been possible to discharge only at times when the Mangatarere was at 3x median flow or

above.

There is agreement amongst freshwater ecologists3 that there are no more than minor effects from

typical treated municipal wastewater effluent for discharges above 2x to 3x median. The Proposed

Natural Resource Plan has no discharge quality limits over 3x median. In this sense, the degree of

3 Consultation discussions with Brian Coffey and Olivier Aussiel

This line represents the point at which

the 30:1 dilution is exceeded – that is

when the dilution is greater because of

the discharge pipe limitations i.e. a

higher discharge would be possible if not

restricted by pipe size.




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additional treatment provided by the sequential batch reservoirs therefore becomes moot, and the

consent application should be based on existing discharge quality to allow for the greatest flexibility

in discharge regime.

12. Preferred Option Notwithstanding the above, Council wishes to provide the best managed outcome not only for the

Mangatarere, but also for the greater Ruamahanga catchment.

Improvements to effluent quality are beneficial in that they would provide a reduction in

contaminant load to the lower Ruamahanga, Lake Wairarapa and Lake Onoke.

With this in mind, Council proposes to instigate a managed discharge regime whereby effluent is

retained in the batch reservoirs for the longest practicable time before discharge, in an attempt to

achieve the best quality of treated water. There are risks with this approach, as the greater the

degree of optimisation of the reservoir use (closer to being full in an attempt to better treat water),

the greater the risk that an extreme event would necessitate a discharge to water at a receiving

water flow of less than 3x median.

Similarly, whilst it is envisaged that the land irrigation system will be more than capable of catering

for summer inflow to the wastewater treatment plant, it is logical to permit discharges at a receiving

water flow of 3x median over the summer if needed, as this prevents the need to discharge at lower

flows. Climate change predictions suggest an increased likelihood of intense summer rainfall events,

and it is prudent to allow for all eventualities.

The ultimate management regime cannot be envisaged at this time as it requires the adaptive

learning of actually applying the discharge in practice. Given the results from the trials, however, it

can be seen that significant improvements in quality are possible over relatively short periods.

Therefore as a starting regime it is proposed to:

Avoid discharges to water in summer wherever practicable

Discharge at river flows over 3x median at a dilution of 30:1 at any time of year as standard

operating procedure. Allow discharge to water at river flows above 2x median when

conditions necessitate.

Target a minimum Sequential Batch Reservoir retention time of 14 days before discharge

Allow for higher irrigation rates to avoid discharges to water in extreme circumstances.

A.Duncan CPEng




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Works Cited Friedler.E. (2003). Simulation model of wastewater stabilization reservoirs. Ecological Engineering.

Greater Wellington Regional Council. (2000). Guidelines for on-site sewage systems in the Wellington

Region.

Juanico, M. (1996). The performance of batch stabilisation reservoirs for wastewater treatment

storage and reuse in Israel. Wat. Sci. Tech.

Juanico, M. (1999). Reservoirs for Wastewater Storage and Reuse. Berlin: Springer.

Juanico, M. (2005). Wastewater reservoirs. In A. Shilton, Pond Treatment Technology. IWA

Publishing.

Pang, L. (2009). Microbial Removal Rates in Subsurface Media Estimated From Published Studies of

Field Experiments and Large Intact Soil Cores. Published in J. Environ. Qual. 38:1531–1559

(2009).

Shilton, A. (2005). Pond Treatment Technology. London: IWA.

Standards New Zealand. (2011). NZS3604:2011 Timber framed Buildings.

Standards New Zealand. (2012). AS/NZS1547:2012 On-Site domestic wastewater management.


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Conceptual Storage and Discharge Option Assessment … · Conceptual Storage and Discharge Option Assessment At Daleton Farm, Carterton District Council ... performance of batch stabilisation