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.
Alternatives to chlorine, and their main attributes are listed below:
ultraviolet – UV light disinfection has been used extensively, but needs a water relatively free from substances that absorb at 254nm to ensure disinfection. Since many reuse facilities employ filtration, this seems a suitable combination. One of the key limitations of UV is the absence of any residual disinfectant following treatment. For many reuse facilities a chlorine residual is desirable to control biological growth in the distribution system. Some end-users also prefer a residual, for example when irrigation water is stored in publicly accessible ponds. UV treated water would have a potential for regrowth of bacteria, as well as development of algae and other undesirable organisms – which a chlorine residual would suppress. The main advantage of UV is the elimination of THM formation and it is one of the few proven technologies to reduce NDMA. But the dosage required for NDMA reduction is about 1,000mJ/cm2, which is significantly greater than the dosage range used in reuse (60-140mJ/cm2) and for the basic level disinfection
(20-40mJ/cm2 recommended by USEPA).
chlorine dioxide – does not promote the formation of THMs and is a highly effective bactericide and viricide. It has been used successfully for years in drinking water disinfection because it minimises the formation of DBPs regulated under the US Safe Drinking Water Act. Similarly, receiving streams would be protected from these byproducts, but as chlorine dioxide tends to be several times more expensive than chlorine, it has not found widespread use as a disinfectant for wastewater.
ozone – has not been widely used for disinfection for similar reasons to chlorine dioxide, even though its efficiency is well documented. Table 1 shows the results of research into the necessary ozone dose for specific disinfection targets. Concentration x time (CT) values for many early facilities ranged from 20-150mg/l min. The most significant factors that influence the ozone dose requirements are effluent soluble COD, influent bacteria density and removal objective.
chloramination – is an effective disinfection method because monochloramine is formed through the reaction of chlorine with ammonia. In an effluent, the available ammonia can be combined into chloramines by chlorine addition or for a fully nitrified effluent, both a controlled amount of ammonia and chlorine can be added. The preferential reaction prevents the formation of non-germicidal organochloramines, which would require extremely high dosages to achieve effective disinfection. Chloramines also minimise the formation of THMs and other DBPs, with the exception of NDMA, which has been identified as a potential byproduct – concentrations of more than 700ng NDMA/l were observed in some tests. One other difficulty is that after dechlorination, monochloramine will be destroyed and ammonia released into the water, leading to possible violations of ammonia discharge standards, and generation of ammonia toxicity.
peracetic acid (PAA) – is a potential new disinfectant, with possible action by active oxygen release or the hydroxyl radical. Regardless of its mechanism, it is an effective disinfectant that is neither mutagenic nor carcinogenic and decomposes into harmless acetic acid, oxygen and water – and therefore does not need dechlorination. On the other hand, the organic content in the treated effluent is increased, as is the potential for biological regrowth. Furthermore, the efficiency for virus and parasite removal is limited and its main use has been with marine discharges for shellfish and bathing water protection, where less stringent limitations are required compared to reuse or recharge.
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.
BCDMH – (1-bromo-3-chloro-5,5-dimethylhydantoin) has traditionally been used for disinfection of swimming pools and cooling towers. Its disinfection action relies on the dissociation of HOBr and HOCl and their oxidisation power. It has recently been developed for stormwater disinfection in Tokyo because the necessary contact time is only a few minutes, compared to
10-20min for economical chlorine doses. Its availability in powder form also facilitates storage for the intermittent use for wet-weather applications. Despite its high halogen content, residual THM, bromate or toxicity seems comparable to hypochlorite applications, but the present experience is not for as stringent pathogen residuals as are common for bathing water (1,000 CFU/100ml) or standard US practice (200CFU/100ml).
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:
receiving stream quality requirements vary depending on the designated use, which will determine environmental regulation, such as bathing water, recreation, water sports, fishing or shellfish farming. The stream designation will govern the degree of disinfection required and concerns over aquatic toxicity, which in turn depends on the disinfection method used, as well as ambient conditions of pH and temperature in the receiving water,
irrigation reuse will require extremely low coliform counts, such as the California Title 22 regulations with its limit of 2.2CFU/100ml. In Arizona, class A effluent standards for unrestricted reuse require counts of fecal coliforms below detection limit in four out of seven samples. A chlorine residual is often required for pipe maintenance and to prevent bacterial regrowth and algae growth in storage basins.
groundwater recharge will require high effluent quality but not involve aquatic life issues. Particular concern on DBPs and THMs need to be addressed and NDMA formation via chlorination has attracted attention in the south-west and west of the US. The presence of organic substances favours THM formation, whereas NDMA is closely related to chlorine dosage and background DMA as well as nitrite concentrations.
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.