Black & Veatch's Frank Rogalla looks at wastewater disinfection techniques
Design variables for determining wastewater disinfection strategies have changed with new technologies, regulations and discharge points. The 'one size fits all' approach no longer applies and economics is no longer the key factor in selection. In addition to compliance with the EU bathing water or shellfish directives and reducing the risks of pathogen infection, some WwTWs must minimise byproducts formation, while also complying with reuse and recharge standards.
Historically, chlorine has been the most common method of disinfection, but its use is under scrutiny due to concerns about transportation and storage, toxicity of chlorine gas and corrosivity of hypochlorite solution. Furthermore, the chlorine residual in a plant effluent can harm aquatic environments and the chlorine addition to wastewater can result in the formation of disinfection byproducts (DBP). Alternative disinfectants such as ultraviolet (UV) light, ozone, chlorine dioxide and chloramines and even membranes are often considered in reuse and recharge applications, primarily to meet regulations on chlorine residuals and DBPs such as trihalomethanes (THM), haloacetic acids (HAA) and nitrosodimethylamine (NDMA). Addressing these problems requires the evaluation of alternative strategies, for instance combining treatments to meet conflicting requirements.Disinfection Options
Alternatives to chlorine, and their main attributes are listed below:
PAA is a relatively expensive chemical and is therefore not competitive for very stringent requirements, such as California standards of 2.2CFU/100 ml or >5 log units inactivation. Under high dosages, long contact times and relevant concentrations of organic and mineral constituents in the effluent, the formation of halogenated byproducts may be a problem, but further investigations into aquatic toxicity and long-term cost considerations are necessary to establish it for a mainstream application.
Disinfection Design Challenges
While disinfection with only a single discharge might be straightforward, optimising the multiple use of effluent for various objectives is more challenging. Possible considerations for the various uses could entail:
Facilities that need to address several objectives simultaneously need to consider the implementation of multiple disinfection strategies, balancing cost issues with the needs of multiple end users. In such cases, a decision model can be developed using Criterion Decision Plus. A decision tree (Figure 1), is developed to represent the issues pertinent to technology selection. This allows for more detailed evaluation of both economic and non-economic factors. Key to the evaluation is the relative importance that is placed on the various criteria. For instance, if a disinfection system is being evaluated for a wet-weather flow application, experience in this particular configuration will have a higher value than conventional uses. Explanatory plots are generated from the decision tree, ultimately providing comparative evaluation scores or value rankings for each alternative included in the analysis. A simplified selection from a complex set of options can be made from this information.