Before starting the optimised design procedure five fundamental questions should have been answered:
- What is the basis of design? For instance, what are the amounts and composition of the waste water to be treated, environmental and climatic conditions etc.?
- What are the costing factors to be used?
- What are the treatment objectives?
- What are the available treatment options?
- What are possible limitations and constraints?
(1) Basis of design
In the design of waste water treatment plants, often neither quantity nor characteristics of the waste water to be treated are known and have to be estimated, for instance based on the size of the population and its expected growth, the fraction of the population served by a sewer system and the expected future development of organic- and hydraulic contributions per capita. For any rational design approach, it will be necessary to attribute values to all of these parameters.
In practice, there will be sometimes be no alternative than to refer to “expert judgment” or “common sense” in order to come up with any numbers at all. In addition to quantitative data concerning the amount and concentration of the waste water, it will be necessary to obtain data about the values of the kinetic parameters of the processes in the future treatment system and of the settleability constants. Click here to download this section
(2) Costing of the activated sludge system
To evaluate the attractiveness of different design alternatives, the relevant costing data are required, which will ultimately define the investment and operational costs. To estimate the total investment costs, indicative cost factors are supplied for the main treatment units. Together with the operational costs, the annualized total investment costs will allow a cost evaluation based on total annual costs, costs per people equivalent and costs per cubic metre of treated wastewater. Click here to download this section. Example 10.1 demonstrates how the annualized investment costs can be calculated.
(3) Treatment objectives
Ultimately, the selected wastewater treatment configuration will depend on the treatment objectives and the legal effluent discharge requirements. Several examples will be discussed. To download this section, click here.
(4) Available treatment options
The optimized design of five basic configurations of the activated sludge system is thoroughly discussed in the handbook. In this section, only three of these configurations will be presented. The other two are listed below in red font.
- Configuration A1: Conventional secondary treatment
- Configuration A2: Conventional secondary treatment with primary settling
- Configuration B1: Anaerobic pre-treatment with aerobic post-treatment (nitrification)
- Configuration C1: Tertiary treatment - Nitrogen removal in a Bardenpho configuration
- Configuration C2: Tertiary treatment - Nitrogen and phosphorus removal in an UCT configuration
From these basic configurations hybrids can be constructed if required. To download this section, click here.
(5) Limitations and constraints
The selected solution in a specific design case depends on legal requirements, but also on other considerations such as the possible reuse of the effluent, the available area for construction, available funds (also taking into account the cost of the sewer system), the environmental and economical impact of the treatment plant and the availability of skilled employees. These will now be discussed. To download this section, click here.