Once the Wholesaler has granted a TE Consent, they will apply a Discharge Point ID (DPID) into the Central Market Operating System (CMOS) for the Retailer to invoice the customer. Note that a customer can have more than one DPID, since it’s possible to have multiple discharge points into the sewerage network.
The DPID applied by the Wholesaler will contain information on how the Retailer should charge the customer for the volume and volumetric rate of their trade effluent. These are the two key aspects of Trade Effluent billing: volume and volumetric rate.
Volume:
Large customers may have a meter that directly records TE volume, but most TE customers just have one meter that measures incoming water supply. This means that the volumes of sewerage and trade effluent need to be assessed or estimated.
For all TE customers, TE volume is calculated using one of three sewerage volumetric adjustment methodologies (SVAMs). Each SVAM has a slightly different calculation, and is set by the Wholesaler based on what they believe to be the most accurate way of invoicing. It should be noted that within each methodology, percentage or fixed allowances can be granted to allow for ‘lost’ volume through trade processes, such as evaporation or absorption. You can see these included in the calculations outlined below.
The three SVAMs are:
The ‘Domestic Allowance’ method involves the Wholesaler estimating the annual volume of domestic sewage for a site. This volume is then deducted from the volume recorded on the water meter to leave only the volume of trade effluent remaining.
The calculation is as follows:
Trade Effluent = Water Volume – Annual Domestic Allowance – Any Allowances
The ‘Subtract’ method involves making use of a Trade Effluent meter. The TE meter volume is subtracted from the Water meter volume to give the domestic sewage volume.
The calculation is as follows:
Sewerage Volume = Water Meter Volume – TE Meter Volume – Any allowances
The ‘None’ method makes use of percentages to help calculate the volume of trade effluent and sewerage. Effectively, the Wholesaler will say what percentage of the incoming water meter volume is sewage, and what is trade effluent. If it is a 50/50 split between the two, the ‘Return to Sewer’ (RTS) and Meter DPID Volume (MDVol) will both be set to 50.
Some customers may choose to install a waste meter to measure the volume directly. Where meters are used for TE volume calculation, estimated meter reads may be used for billing purposes. The estimates used can be based on previous meter reads or, in the absence of any reads, can simply be an Industry Level Estimate. Detail on Industry Level Estimates can be found on the market operator, MOSL’s, website. Without metered volumes, the wholesaler will provide an estimated annual volume based on the business activities.
If you would like to query which volume methodology is used to calculate your invoicing, you can contact trade.effluent@castlewater.co.uk.
Volumetric Rate:
Volumetric rate refers to the rate charged per unit of TE volume. The volumetric rate for TE is different to those of water and sewage in that it is variable; it can change depending on the Suspended Solids (mg/l) and Chemical Oxygen Demand (mg/l) strengths within the TE discharge. The Wholesaler will take samples of the TE discharge from a site, and this sampling will inform the volumetric rate. Depending on how large the site is, the Wholesaler may sample every few weeks, meaning the rate at which trade effluent volume is charged can fluctuate considerably.
The formula used to calculate the TE volumetric rate is the Mogden Formula. This formula makes use of individual unit charges to give a combined trade effluent rate, as shown below:
Charge Per Unit of Effluent (£/m3) = R + [V or M] + B(Ot/Os) + S(St/Ss)
Where:
R = Reception and Conveyance Charge (£/m3)
V = Primary Treatment Charge (£/m3)
M = Treatment and disposal charge should effluent go out to Sea (£/m3)
B = Biological settled Sewerage Charge (£/m3)
Ot = Chemical Oxygen Demand of effluent after one hour of quiescent settlement (mg/l)
Os = Average Chemical Oxygen Demand strength (mg/l)
S = Treatment and disposal of primary sewage sludge charge (£/m3)
St = Total suspended solids of effluent (mg/l)
Ss = Average suspended solids strength (mg/l)
When the Wholesaler samples the discharge point, they obtain the Ot and St values of the effluent; these are the components within the Mogden formula that can lead to changes in the unit rate of effluent over time. The higher the measured chemical oxygen demand and suspended solid content within the effluent, the higher the TE Volumetric Rate will be. In other words, the dirtier the discharge, the higher the bill.
The remaining components of the formula are fixed for the financial year and are detailed in Wholesaler and Retailer Scheme of Charge Documents. Castle Water’s default Scheme of Charges can be found here: https://www.castlewater.co.uk/help-support/scheme-of-charges.
Depending on the volume of discharge, a tariff will be applied to each discharge point detailing the unit rate at which each TE component is invoiced. Tariffs are split into several different ‘bands’ corresponding to the annual chargeable volume that the tariff applies to. The Wholesaler determines which tariff applies for each water services component. Further information on this can be found in the Scheme of Charges.
Castle Water Invoicing:
TE customers with a long-term consent will have their charges for TE included within their invoices for water and domestic sewage. Castle Water typically bill all TE customers on a monthly in-arrears basis.
For customers with short-term consent, a one-off charge will apply for the treatment and disposal of effluent from trade discharges into the public sewer system. These fees vary and are based on the volume discharged under the consent as well as the chemical composition of the effluent. These charges will be invoiced upon the wholesaler raising the charge.
Where there is change to billing data, such as a new meter read or an update to TE sample data, Castle Water will provide an adjustment to previous invoicing. This adjustment will show the difference between the charges we have previously issued and the recalculated charges, and prevents customers having to receive multiple credit notes and invoices when changes occur in billing data. The most common reason for an adjustment invoice is a new meter read.