CHAPTER 13
Introduction
GPS-X contains operating cost models for most of the objects in the process table. Each object can be set up to calculate costs for energy, chemical dosage and sludge handling (as appropriate to that object).
This chapter discusses the structure of the operating cost models, and their use and calibration.
Model Structure
For each of the wastewater unit process objects in GPS-X, a set of operating costs has been assigned. These costs reflect the typical operating costs associated with that particular unit process.
GPS-X can be used to dynamically simulate these operating costs in the same way that it dynamically simulates the wastewater unit process itself.
The different types of operating costs modelled are:
· Aeration energy cost
· Pumping energy cost
· Mixing energy cost
· Heating energy cost
· Other miscellaneous energy cost
· Chemical dosage cost
· Sludge handling cost
The following sections described each cost in detail.
Aeration Energy Cost
An aeration energy cost can be calculated for unit processes that have aeration, such as the CSTR, plug flow tanks, SBR, etc. The amount of energy required to supply the calculated level of aeration depends on several factors, including blower/compressor efficiency, headloss, etc., and the factors included in the oxygen transfer model (See Modelling of Oxygen Transfer in Chapter 6).
GPS-X can simulate two different aeration methods - mechanical and diffused air. If mechanical aeration is chosen, an aeration power (kW) is entered. If diffused aeration is chosen, the power requirement is calculated as shown in Equation 6‑27 and Equation 6‑28 in Chapter 6.
The wire power is multiplied by the energy price ($/kWh), and then integrated over time to determine the total cost for aeration energy during the simulation
Blower Energy Cost
Energy required for the pumping of water and wastewater is modelled in GPS-X using the following equation:
Equation 13‑1
Equation 13‑2
Equation 13‑3
Where:
= Blower power, kW
= Air temperature, ºC
= Air Flow rate, m3/d
= Air pressure at discharge, kPa
= Air pressure at the inlet, kPa
= Blower efficiency, -
= Atmospheric pressure, kPa
= inlet
pressure loss, kPa
= diffuser submergence, m
= head loss in piping and diffuser, kPa
The energy value is multiplied by the energy price ($/kWh), and integrated over time to determine the total cost for aeration energy during the simulation.
For blower energy costs, the head value represents the sum of the actual head and the piping headloss. There is no additional equipment headloss as with aeration energy costs.
Mixing Energy Cost
The mixing energy cost model describes the use of energy for mechanical mixing operations.
Users are required to input the required mixing power usage per unit volume (kW/m3). This value is multiplied by the volume of the tank, and then multiplied by the energy price ($/kWh) and integrated over time to determine the total cost for the simulation.
Heating Energy Cost
The heating energy cost model describes the use of energy for heating individual unit process operations such as digesters, or activated sludge units in cold environments.
Users are required to input a heating power usage per unit volume (kW/m3). This value is multiplied by the energy price ($/kWh) and integrated over time to determine the total cost for the simulation.
Other Miscellaneous Energy Cost
The miscellaneous energy cost model describes the use of energy for various mechanical operations such as gates, arms, rakes, moving bridges, etc.
Due to the complex nature of estimating these types of energy requirements, users are required to input a flat rate of energy usage (kW). This value is multiplied by the energy price ($/kWh) and integrated over time to determine the total cost for the simulation.
Chemical Dosage Cost
Some unit process objects in GPS-X allow for chemical addition as part of treatment (some influents, equalization tank, DAF, dewatering units, etc.).
In each case, the total chemical dosage cost is:
Equation 13‑4
where:
chemicalcostperday = chemical dosage cost ($/d)
chemprice = chemical price ($/kg)
chemdosagerate = hydraulic head (kg/d)
This daily cost is integrated over time to determine a total cost for the simulation.
Sludge Disposal
Several objects in GPS-X have connection points which represent thickened or dewatered sludge (e.g. dewatering unit). A sludge disposal cost can be associated with these flows to determine a sludge handling cost.
The cost is determined by multiplying the per-unit disposal cost by the rate of sludge disposal:
Equation 13‑5
where:
disposalcostperday = sludge disposal cost ($/d)
disposalcost = disposal price ($/m3)
sludgerate = sludge disposal rate (m3/d)
This daily cost is integrated over time to determine a total sludge disposal cost for the simulation.
Operating Cost Model Parameters
Operating cost parameters are found in two places:
General Data Forms: Right-click on an open spot in the layout. These forms contain general operating cost parameters that apply to the entire layout (i.e., energy prices and schedules).
Operating Cost Parameter Forms (in each object): These forms contain operating cost parameters that are specific to each object (i.e., headloss, pump efficiency, chemical cost).
General Operating Cost Parameters
Operating cost variables controlling the price
of energy can be found in the
Layouts > General Data > System > Input Parameters
> Operating Cost Settings menu
(See Figure
13‑1).
Figure 13‑1 – General Operating Cost Parameters Form
The price of energy can be set in two different ways, by selecting from the Energy Pricing menu:
· Time-based Pricing (energy price varies throughout the day)
· Seasonal Pricing
When Constant Price is selected, the energy price is set to the value entered on the form. This value can be changed in a scenario or by placing the price on an interactive controller, or set constant for the entire simulation.
When Time-based Pricing is selected, the energy price will cycle through a user-defined set of prices on a user-defined schedule. The number of different energy prices is unlimited. This mode can be used to simulate lower energy costs during the night.
The Seasonal Energy Pricing model allows the user to evaluate the cost of power consumption under a dynamic price structure based on season, weekday, weekend and hour of the day. The model is an extension of the Time-Based Pricing model which only allows specification of daily variation in energy price. The start day and month for each season is required to be set by the user.
In seasonal pricing model, two seasons of summer and winter are set each of which can have different energy pricing structure. Three tiers of off-peak, mid-peak and on-peak prices can be specified by the user.
In each season, weekdays and weekends are allowed to have any hourly price structure using the three tiers of pricing level.
The input form for the seasonal price model is as shown below.
Figure 13‑2 – Input menu for seasonal price model
In seasonal price model, date and time set in the Simulation Setup menu is used to determine the season and day name. This information is used to determine the energy price based on the user inputs.
Object-Specific Operating Cost Parameters
For all objects, the operating cost model parameters can be found in the Parameters > Operating Cost menu, and its associated More... button. The values entered on these forms are specific to each object. These objects can be tagged and placed on controllers, etc., as with any other parameter.
Figure 13‑3 - Operating Cost Menu
The operating cost model parameters shown in the Operating Cost parameters form depend on the object and choice of model.
Operating Cost Model Output Variables
Similar to the operating cost parameters, the associated output variables are divided into general and object-specific groups.
General Operating Cost Output Variables
Operating cost output variables can be selected from the Layout > General Data > System > Output Variables > Operating Cost menu. These output variables include the energy price; total energy, chemical and sludge disposal costs for the entire layout; and the total operational cost for the layout (sum of the total energy, chemical and sludge disposal costs).
Figure 13‑4 - Layout Operating Cost Display Form (total for all objects)
Object-Specific Operating Cost Output Variables
For all objects, operating cost model output variables can be selected from the Output Variables > Operating Cost menu by right clicking on the effluent connection point.
The output variables available include energy usage and sludge disposal rates as well as total energy, chemical and disposal costs for that particular object. The specific costs calculated (and available in this menu) depend on the operating costs associated with the particular object and model.
Figure 13‑5 - Object-Specific Operating Cost Display Form
Calibration of Operating Cost Models
The operating cost models in GPS-X are designed to simulate typical operating costs found in typical wastewater treatment facilities. The models are populated with parameters that give costs for typical plants.
To calibrate the models to the behavior of the plant being simulated, the user will need to adjust various parameters in the models. This list can be useful in guiding calibration.
Aeration
Enter the hydraulic head and headloss for the system, if known. Otherwise, use the defaults. Calibrate with the blower efficiency and diffuser headloss.
Pumping
Enter the hydraulic head and headloss for the system, if known. Otherwise, use the defaults. Calibrate with the pump efficiency and/or piping headloss
Chemical Dosage & Sludge Disposal
As these calculations are a flat “flow multiplied by price” calculation, the model is calibrated by adjusting the price parameter (flow is calculated or set elsewhere).
Mixing & Heating Energy
As these calculations are done on a “per unit volume” basis, make sure that you have the correct volume for the unit. Adjust the power per unit volume usage as needed.
Miscellaneous Energy
The miscellaneous energy uses are flat rates, so they are calibrated by adjusting the rates as appropriate.