treatment line design: limitations

The array of possible treatment lines combining the processes and technologies already described in chapters fundamental biological engineering processes applicable to water treatment and biological processes, is extensive, especially when these are combined with sludge treatment lines (see general sludge treatment lines in figure 1).

Nevertheless, the designer of a municipal wastewater treatment plant WWTP project will notice the choices becoming narrower as the different “local” limitations are taken into consideration along with a fundamental objective: to achieve a minimum overall cost (amortising capital investment and operating costs).

This overall cost is traditionally called TOTEX (Total Expenditure). Taking into account the initial CAPEX (Capital Expenditures) and OPEX (Operational Expenditure) in a facility's life cycle cost, the use of this global approach enables significant optimisation drivers to be identified which can often provide attractive savings and returns on investment.

Significant experience of water treatment infrastructures and processes allows better identification of the drivers and needs, which the optimisation of industrial design engineering benefits from. Putting these design, construction and operating costs into perspective throughout a facility's entire service life highlights the increasing interest in continually improving energy efficiency and the need for more detailed assessments of the WWTP’s global environmental footprint.

The concept of minimum global costs and life cycle cost emphasises the growing interest for (Design Build Operate) and BOT (Build Operate and Transfer) type contracts versus DB (Design and Build) or shopping 'list' contracts, traditionally deployed throughout the world.

Of these limitations, the following are the most important and the most recurrent:

• the characteristics and the variability of the effluent to be treated;
• ability to reliably achieve the target treated water quality;
• the intended end-use of the sludge produced by the plant;
• the plant's environment;
• construction layout options;
• the land available;
• the desire to allow for sustainable development;
• if applicable, the refurbishment of all or part of an existing plant with the major construction scheduling problems this involves.

Note that this list is not exhaustive.

This sub-chapter addresses the limitations involved and their impact. In section choice of waterline technologies the technologies that best meet the objectives set are reviewed.