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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
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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
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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
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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
Storage and discharge options April 2017 V9
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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.