4. Dosing options

4.1 Introduction

There are three main forms of dosing: Rainfall Activated; Flow Activated; and Batch Dosing.

In the rainfall activated process during rainfall the dosing system is automatically activated by rainfall to provide the appropriate volume of additive to the sediment laden surface runoff.

In the flow activated process the dosing system is automatically activated by flow (typically pumped) to provide the appropriate volume of additive to the sediment laden inflows.

The batch dosing process is typically undertaken to treat an impounded volume of sediment laden water, typically as the result of pumping. This is a manual process where the volume of impounded water is determined and from this the appropriate volume of additive determined, added and mixed.

The sampling and testing of soils detailed in Section 3 of this FTMP will be utilised to determine the appropriate chemical and dose rate.

4.2 Rainfall activated dosing

4.2.1. Introduction

Rainfall activated dosing will be utilised where the sediment retention devices receive runoff as a gravity surface flow. This system will typically be used in the early stages of the works with a transition to flow activated dosing as the excavation progresses and gravity surface flow decreases.

The flocculation systems to be used will be designed in accordance with GD05.

The basic design will be a rainfall activated system (typically) using Polyaluminium Chloride (PAC) to dose the Sediment Retention Pond (SRP).

The layout of a typical PAC Rainfall Activated System is shown in Figure 4-1 below.

Figure 4-2: Typical PAC Rainfall Activated System Layout

4.2.2. Rainfall catchment tray

The rainfall catchment tray will be sized to provide treatment at the design dose rate for 100% of the runoff from exposed earthwork areas and for 60% of the runoff from any stabilised areas within the catchment.

A 50mm storm event will be used for calculating the size of the tray.

4.2.3. Header tank

The header tank design provides for 12mm of rainfall before dosing commences.

At this point a low rate outlet hose with a 3mm orifice is installed.

The high rate outlet hose with a 10mm orifice is installed at the level reached by a further 12mm of rain above the low flow outlet.

The header tank has a minimum freeboard of 50mm above the high rate outlet.

The header tank typically has a diameter of 580mm (a recycled PAC drum).

4.2.4. Displacement tank

The displacement tank will be a neat fit inside the flocculent reservoir tank.

The minimum displacement tank capacity will be the equivalent of a 100mm rainfall event.

4.2.5. Flocculent reservoir

The flocculent reservoir tank will have a minimum capacity to store sufficient flocculent to dose runoff from a 100mm rainfall event.

4.3 Flow activated dosing

4.3.1. Introduction

Flow activated dosing will be utilised where the sediment retention devices receive runoff as a pumped flow. This system will typically be used in the mid to later stages of the works as the excavation progresses and gravity surface flow decreases.

Flow activated dosing will be utilised as part of a Site Water Treatment Plant (SWTP). Subject to the activities taking place on site and the contaminants encountered, more than one SWTP may be utilised at any one time.

The use of and specific design of each SWTP is detailed within the applicable SSESCP.

4.3.2. Site Water Treatment Plant(s)

The SWTP will be a treatment device that allows for:

Pumping sediment laden runoff captured within the site into a storage tank(s) (such as a 75m 3 modified shipping container(s) or similar).

The system will buffer flow volumes, allow for initial settlement of sediments and any chemical treatment (i.e. pH adjustment, flocculation). The discharge will be piped to the stormwater network, unless monitoring has shown that contaminant levels prevent discharges to stormwater. In this event discharges will be directed to the wastewater network in accordance with the Trade Waste Agreement (to be obtained). The SWTP will be manned during operation, ensuring that in the event of discharge standards being exceeded, the transfer pump will be stopped and the discharge will cease until the required water quality parameters can be met. The water can be recirculated from the settlement tank to the buffer tanks using submersible pumps if required.

The SWTP will be located in a suitable location to provide connections to power and a connection to the stormwater and wastewater network.

4.3.3. Site Water Treatment Plant operation

The SWTP will be operated in the following manner:

Inflows from the site will be pumped to the buffer tanks, which are the primary storage tanks.

Typically flows will be pumped from the site to the buffer tanks as fast as possible so as to keep the site as dry as possible.

From the buffer tanks water will be pumped to the settlement tank as follows:

Inflow to the settlement tank will be measured by a flow meter, which in turn will trigger flocculent dosing and pH adjustment dosing as required (see below).

In order to maximise the efficiency of the settlement tank, it is anticipated that flocculant dosing will be beneficial. The dosing will be undertaken as follows:

The specific flocculant to be used and dose rate will be determined by bench testing that will be undertaken during setup of the water treatment plant and prior to commencing operation.

As works progress additional testing will be undertaken as required by changing soil types.

The flocculant dosing will be undertaken by an automated pump, triggered by inflow rates which will add the flocculant at the required rate.

To comply with the WQDMP pH adjustment will also likely be required and undertaken as follows:

pH adjustment will be undertaken using citric acid or similar product, dosed using an automated pump triggered by inflow rates.

The dose rate to buffer the pH will be determined by a pH sensor, which will sample water within the settlement tank and will be adjusted by the SWTP operator as required.

From the settlement tank, compliant outflow will be discharged to the stormwater network. The SWTP operation and discharge quality will be monitored daily by the SWTP plant operator. Where this discharge quality is identified as non-compliant for stormwater discharge the discharge will be recirculated for additional treatment. If this is impractical and the flow is compliant for trade waste disposal the discharge will be directed to the wastewater network. Maintenance of the SWTP will include the following:

Regular cleaning of pH sensors;

Top up of flocculant and pH buffer; and

Regular removal of accumulated sediment within the buffer and settlement tanks (before the total storage volume of the tanks has been reduced by 20%).

4.4 Batch dosing

4.4.1. Introduction

Batch dosing will be undertaken where runoff is impounded and may be the result of ineffective rainfall activated or flow activated dosing. It will also be utilised for minor flows

4.4.2. Dosing trigger level

The trigger level for batch dosing will be 100mm clarity, i.e., when the clarity of impounded sediment laden runoff falls below 100mm batch dosing will be undertaken in accordance with the following batch dosing methodology.

4.4.3. Batch dosing methodology

The outlet of any impounded runoff will be blocked. This may be done either with a pre-installed valve or a crimped (folded) section of lay flat hose connected to the outlet of the decant or other methods such as raising the decant.

Following rainfall, the clarity will be checked and if less than 100mm, the required chemical will be added by spraying it on the surface of the SRP or by careful application using a bucket or other small container to evenly disperse the chemical over the surface of the SRP.

The chemical will be added at the standard rate of dosing for the devices on site, this rate expressed as a litre of chemical per 10m 3 of impounded runoff will be included within the specific FMTP details

for that device.

The impounded water will then be mixed with the chemical using one of the following methods:

Mixing with a pole, paddle or oar; Mixing by circulating the impounded flows through a pump; or Mixing by dragging semi-submerged floats through the surface area of the impounded water.

Once settlement has occurred, typically within 2-3 hours the pH and clarity within the device will be checked. If the pH is between the range of 5.0 and 9.0 and the clarity is greater than 100mm the decant will be opened and the device drained.

If the clarity is less than 100mm a further dose of chemical will be added at 50% of the original rate and a further settlement period of 2-3 hours allowed before retesting.

If the pH levels fall outside the acceptable range of 5.0 to 9.0 the Environmental and Sustainability Manager will be contacted to determine appropriate actions which could include off site removal of impounded water or adding alternate chemicals, such as sodium carbonate or sodium bi-sulphate, to alter the pH.

Subject to the weather conditions at the time and the level by which the range has been exceeded it may be appropriate to use the impounded water for dust control.