Page 12: Canadian Guidelines for Domestic Reclaimed Water for Use in Toilet and Urinal Flushing
Water reclamation typically makes use of conventional wastewater treatment technologies that are widely used and readily available. The discussion of treatment for reclaimed water focuses largely on whether the treatment system is capable of consistently achieving an appropriate water quality. Most international examples of guidelines for the use of recycled water specify both general treatment processes and water quality limits for a particular group of applications (Bahri and Brissaud, 2003).
Overview of wastewater treatment for reclaimed water
The treatment of wastewater is usually performed by a combination of biological, physical and chemical processes. Biological treatment uses microorganisms in suspension in the wastewater or attached onto a support media, to assist in the removal of matter from the wastewater. Physical treatment removes the waste by filtration through a granular media or through a solid media, such as membrane filtration. Chemical treatment involves adding specific chemicals to precipitate targeted components or adsorbing them onto a media. All of these processes can provide different degrees of treatment. The terms widely used to describe these degrees of treatment, in order of increasing treatment level, are primary, secondary, advanced secondary and tertiary treatment. The definitions of these treatment levels vary. The definitions and descriptions provided in this appendix are for the purposes of this document only. Wastewater treatment levels considered suitable for the purposes of producing reclaimed water for toilet flushing use in residential and commercial buildings include secondary, advanced secondary and tertiary treatment systems. These are typically characterized by the water quality produced in terms of biochemical oxygen demand (BOD) and total suspended solids (TSS) concentrations and the degree of nitrification achieved in converting ammonium to nitrate. Table D1 provides an overview of indicative removals of microbial hazards that can be achieved using various treatment processes and treatment levels.
|Treatment||Indicative log reductionsTable D1 Footnote b|
|E. coli||Bacterial pathogens||Viruses||Giardia||Cryptosporidium|
|Membrane filtration||3.5-> 6.0||3.5-> 6.0||2.5-> 6.0||> 6.0||> 6.0|
Primary treatment removes coarse organic and inorganic solids and grit by sedimentation and/or flotation. The organic contaminants removed can represent a significant portion of the overall BOD, TSS and fats, oils and grease in the raw wastewater. Some of the nitrogen and phosphorus may also be removed, but this is typically not an objective of primary treatment. Primary treatment alone is not sufficient to generate reclaimed water of an acceptable quality. It is, however, an important step to conduct before most secondary and advanced secondary treatment processes.
The principal purpose of secondary treatment is to remove the soluble organic components of the wastewater, in addition to colloidal or suspended forms, following primary treatment in a septic tank for smaller decentralized or on-site treatment systems. Treatment benefits include the removal of residual particulate material, inorganic contaminants and pathogens that are adsorbed (attached) to the biosolids within the system.
Secondary treatment includes an array of biological processes and requires an environment within the treatment system that is suitable for rapid microbial growth. Since aerobic (oxygen-consuming) bacteria treat wastewater more quickly and efficiently than do anaerobic (no oxygen) bacteria, secondary treatment typically involves aerobic bacteria. This means that oxygen must be provided to the system either passively, through the diffusion of air through the system (as is the case with sand filters), or mechanically, introduced using blowers.
After secondary treatment, the effluent typically has BOD5 and TSS concentrations less than 30 mg/L and can be effectively disinfected. Organic contaminants that are resistant to microbial breakdown, nutrients and residual solids may remain in the wastewater effluent after secondary treatment.
Advanced secondary treatment (an alternative to secondary treatment)
In advanced secondary treatment, the same treatment processes and technologies described for secondary treatment are followed by filtration to remove residual and colloidal solids and some additional BOD.
Advanced secondary treatment refers to systems that can reliably achieve effluent quality approaching the detection limits for BOD5, TSS and (with disinfection) thermotolerant coliforms. The effluent from advanced secondary treatment systems is expected to have BOD5 and TSS concentrations less than 10 mg/L. Filtration is included in the treatment process when efficient disinfection is required. This level of treatment is often used internationally in standards or guidelines for
"unrestricted public access" reclaimed water use.
"Unrestricted public access" applications typically include recreational water uses, playing field irrigation, landscape impoundments, direct discharge to streams, vehicle washing, etc.
Tertiary treatment refers to further removal of colloidal and suspended solids, as well as nutrient (phosphorus and nitrogen) removal from the wastewater by either biological or chemical means. Nitrogen released to surface water can be a factor in nuisance algal growth and, if released in the form of ammonia, can be toxic to aquatic organisms.
Nutrient removal can be achieved in a number of ways, including biological and chemical treatment. Biological treatment is generally carried out using an activated sludge (suspended growth) treatment process, which has been compartmentalized into
"environmental" zones, and in which bacteria can be conditioned to remove nitrogen or phosphorus. Treatment systems capable of removing nutrients biologically are more complex and require greater operator skill and attention as well as considerable engineering design input.
In chemical treatment, phosphorus can be precipitated by adding specific chemicals to the wastewater or by adsorption through a special filter. Ammonia can be removed with ion exchange resins or with zeolite. However, chemical addition is not generally considered practical for small wastewater treatment applications. The simple conversion of ammonia to nitrate using dissolved oxygen (i.e., nitrogen conversion but not removal) is also sometimes referred to as tertiary treatment. Although nitrogen is not effectively removed, the ammonia concentration in the effluent (and thus the potential aquatic toxicity) is reduced.
Disinfection is an essential treatment component of almost all wastewater reclamation applications. Disinfection destroys or inactivates the majority of microorganisms within the treated wastewater effluent, including those that are pathogenic to humans. There are three commonly applied methods of disinfection. These are 1) chlorine and alternatives (chlorine dioxide, chloramines); 2) ozonation; and 3) ultraviolet (UV) irradiation. Many disinfection technologies are available and can be designed for treatment applications ranging in size from small on-site to large-scale treatment applications. Although there are exceptions, treated effluents intended for use as reclaimed water will generally require filtration in order to enhance the impact of disinfection processes. Table D2 provides ranges of indicative log removals for enteric pathogens and indicator organisms. Tables D3 and D4 provide a comparison of the concentration (mg/L) and time (minutes) (CT) values for various degrees of virus and Giardia inactivation in water, for the methods of disinfection described in this section (chlorine, chlorine dioxide, ozone). Table D5 provides information on UV light dose for these same organisms as well as for Cryptosporidium. Note that the CT values and UV doses were developed for water of specific characteristics and not for domestic wastewater. Also, the CT values shown for chlorine are based on having a free chlorine residual.
|Treatment||Indicative log reductionsTable D2 Footnote b|
|E. coli||Bacterial pathogens||Viruses||Giardia||Cryptosporidium|
Table D2 Footnotes
|Ozonation||2.0-6.0||2.0-6.0||3.0-6.0||0.5-3.0Table D2 Footnote c||0.25-3.0Table D2 Footnote d|
|UV light||2.0-> 4.0||2.0-> 4.0||> 1.0 adenovirus
> 3.0 enterovirus
|> 3.0||> 3.0|
|2 log||3 log||4 log|
Table D3 Footnotes
|ChlorineTable D3 Footnote b||3||4||6|
|Chlorine dioxideTable D3 Footnote c||4.2||12.8||25.1|
|0.5 log||1 log||1.5 log||2 log||2.5 log||3 log|
Table D4 Footnotes
|ChlorineTable D4 Footnote b||17||35||52||69||87||104|
|Chlorine dioxideTable D4 Footnote c||4||7.7||12||15||19||23|
|OzoneTable D4 Footnote c||0.23||0.48||0.72||0.95||1.2||1.43|
Table D5 Footnotes
Biosolids and residuals treatment
Biosolids treatment involves the treatment of solids that settle out during either or both of he primary and secondary wastewater treatment processes. The requirements for treatment or disposal of biosolids may vary between jurisdictions. Depending on the size of the treatment facility, the primary solids may be stored and hauled away (e.g., septic tank) or transferred to a digestion facility to be stabilized prior to disposal. Digestion may be carried out by bacteria aerobically (with oxygen) or anaerobically (without oxygen), the former being a faster stabilization process but requiring more power, and the latter being a slower process that can be used to generate methane gas (biogas) for power generation if at an appropriately large enough scale. Alternative means of organic solids stabilization include composting and incineration.
Selection of appropriate treatment levels or scale
Wastewater can be treated on site, at the home or building where it is generated, or it can be transported via a sewer to a common wastewater treatment or reclaimed water treatment plant. Studies of centralized facilities have shown that wastewater treatment processes are capable of significantly reducing the numbers of pathogens or indicator organisms present in wastewater, although removal efficiencies will vary with the treatment process type, retention time, oxygen concentration, temperature and the efficiency in removing suspended solids (Garcia et al., 2002; Koivunen et al., 2003; Scott et al., 2003; Rose et al., 2004). In one study, a full-scale municipal treatment plant using biological treatment, filtration and chlorination was shown to reduce total and faecal coliforms by > 7 log and coliphages and enteric viruses by > 5 log. Protozoan pathogens (Giardia and Cryptosporidium species) were reduced by more than 3 log (Rose et al., 1996). While filtration has been found to be the most effective treatment process (in a conventional treatment train) for removing protozoan cysts and oocysts, infectious Cryptosporidium oocysts are detected even in the final effluent from facilities that use filtration processes (Gennaccaro et al., 2003; Scott et al., 2003; Rose et al., 2004). Monitoring data from Florida facilities indicate that, in general, the facilities that have reported pathogen data have been well operated (based on TSS, turbidity and total chlorine residual measurements). Some of the Florida facilities reporting the highest concentrations of pathogens in treated water appeared to provide effective filtration and disinfection. The range of Giardia cysts reported as potentially viable was 10-90% (average 61%), whereas the viable fraction of Cryptosporidium ranged from 70% to 90% (average 77%) (York et al., 2003). These findings suggest that although effective treatment of wastewater will produce a high quality of effluent, it is likely that some risks from viable pathogens will remain.
Over the last 20 years, many of the processes found in centralized treatment systems have been incorporated into on-site systems. The result has been improved system performance and wider-scale acceptance of the on-site wastewater treatment concept. New technologies that are capable of advanced secondary treatment are becoming available for on-site applications suitable for water reuse consideration (Chu et al., 2003; Diaper, 2004). Ranges in treatment performance are shown, as even an optimized system will show some variability in treatment performance. The information in Table D1 and D2 can be used to characterize risk in a simple, deterministic process such as that described in Section 4.5 and Appendix B. However, to characterize risk more accurately, it is preferable to use information that is specific to a given system designed to address the local or unique conditions of the installation. As an example, membranes come with a relatively wide range of pore size, which will have different performance expectations.
There are relative advantages and disadvantages to every type of treatment technology, regardless of the scale of application. Some processes are better suited to on-site needs, whereas others are better suited to more centralized applications. Those technologies that are mechanically complex or require greater operator attention are better suited to centralized facilities where skilled personnel are available. Processes of this kind can be broadly referred to as intensive systems that offer high performance but require a high degree of inputs, such as power, process control and operator skill level. Alternatively, processes that have fewer operating controls or variables, or where few skills are required to operate and maintain the system, are generally better suited to on-site applications.
Report a problem or mistake on this page
- Date modified: