March 13, 2017
The designations employed and the presentation of material throughout this publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The ideas and opinions expressed in this publication are those of the authors; they are not necessarily those of UNESCO and do not commit the Organization.
Verbyla, M., von Sperling, M. and Maiga, Y. (2017). Waste Stabilization Ponds. In: J.B. Rose and B. Jiménez-Cisneros (eds), Water and Sanitation for the 21st Century: Health and Microbiological Aspects of Excreta and Wastewater Management (Global Water Pathogen Project). (J.R. Mihelcic and M.E. Verbyla (eds), Part 4: Management Of Risk from Excreta and Wastewater - Section: Sanitation System Technologies, Pathogen Reduction in Sewered System Technologies), Michigan State University, E. Lansing, MI, UNESCO. https://doi.org/10.14321/waterpathogens.65
Acknowledgements: K.R.L. Young, Project Design editor; Website Design: Agroknow (http://www.agroknow.com)
|Last published: March 13, 2017|
Waste stabilization ponds (WSPs) are sanitation technologies that consist of open basins that use natural processes to treat domestic wastewater, septage, and sludge, as well as animal or industrial wastes. They can be used in centralized or semi-centralized sewerage systems, they can also be used to treat fecal sludge from onsite dry sanitation systems, or as onsite water-based sanitation systems serving a single building or home. The most common types of WSPs are anaerobic ponds, facultative ponds, maturation or polishing ponds, aerated ponds, and high-rate algal ponds (HRAPs). Some pathogen removal is accomplished in anaerobic, facultative, aerated ponds and HRAPs, even though their primary function is to remove and stabilize organic matter. The primary function of maturation and polishing ponds however, is to remove and inactivate pathogens. Under optimal conditions, removal efficiencies in full-scale WSP systems with several units in series can be as high as 6 log10 for fecal bacteria and 4 log10 for viruses, protozoan (oo)cysts, and helminth eggs, however the efficiency of pathogen removal in full-scale systems is highly variable, and in practice many WSP systems achieve only 2 to 3 log10 removal. Some of the most important factors influencing pathogen removal efficiency in WSPs include hydraulic retention time and efficiency, water clarity, pond depth, sunlight exposure and penetration, temperature, and pH. Shallow (<1m) baffled maturation ponds with low turbidity, high pH, and plenty of sunlight exposure will achieve the most efficient pathogen reduction. The sludge/sediments from WSPs (especially anaerobic, facultative and aerated ponds) must be removed periodically, and treated or managed appropriately to limit human exposure. The concentration of viable helminth eggs and protozoan (oo)cysts in this sludge can be as high as 2,000 – 4,000 per gram of total soilds, and helminth eggs in particular can survive in WSP sediments for years. WSP sysetms require large areas of open land, making them ideal in smaller towns and rural settings, though they are used successfully in many urban environments as well, often in combination with other sanitation technologies. One of the biggest advantages of WSPs is that they are easy and inexpensive to operate and maintain, and generally do not rely on mechanized equipment or expensive material or energy inputs.
Waste stabilization ponds (WSPs) are open basins enclosed by earthen embankments, and sometimes fully or partially lined with concrete or synthetic geofabrics. They employ natural processes to treat domestic wastewater, septage, and sludge, as well as animal or industrial wastes. They can be used in centralized or semi-centralized sewerage systems, serving cities or towns; they can also be used as onsite systems serving a single entity (e.g., highway rest area, community center, etc.) (Figure 1). WSPs are frequently used in combination with other sanitation technologies. The most common types of WSPs are anaerobic ponds, facultative ponds, maturation ponds, aerated ponds, and high-rate algal ponds (HRAPs). These ponds differ in terms of their function in the overall wastewater treatment system. The main function of anaerobic, facultative and aerated ponds is the removal of carbon-containing organic matter, while the main function of maturation ponds is the removal of pathogens. HRAPs were developed to optimize the efficiency of organic matter removal while simultaneously allowing for the recovery of dissolved nutrients that become incorporated into the algal biomass. These different pond types are distinguished from each other by their depth, hydraulic and organic loading rates, and by whether or not they use mechanized equipment for mixing or aeration. In general, anaerobic ponds are deepest (≥3.0 m) and are used first in series; facultative ponds are shallower (1.5 – 3.0 m) and may be used first or second in series (after anaerobic ponds); maturation ponds are shallowest (≤1.5 m), and are used last in series. Aerated ponds may be used anywhere in a series of ponds, and HRAPs are often used in by themselves or between anaerobic and maturation ponds. For more information about the design of WSP systems, refer to von Sperling (2007), Oakley (2005), Shilton (2006) or Mara (2003). Figure 2 illustrates different types of ponds and one schematic of a typical WSP system design with three different types of ponds (anaerobic, facultative, and maturation) operating in series.
Figure 1. Waste stabilization ponds are a centralized or semi-centralized treatment technology in the overall sanitation service chain, used with sewered sanitation systems and also to treat the contents of onsite systems
Figure 2. Schematic of a typical waste stabilization pond system (top); anaerobic pond in Brazil (middle left) (photo from Stewart Oakley); facultative pond in the United States (middle right) (photo from Matthew Verbyla); maturation pond in Bolivia (bottom left) (photo from Matthew Verbyla); high-rate algal pond in Burkina Faso (bottom right) (photo from Ynoussa Maiga)
WSPs can be used to treat a variety of water and waste streams, thus the inputs may include wastewater, septage, latrine pit contents, and/or sludge from other wastewater treatment processes (Figure 3). Some WSP systems also receive landfill leachate. WSPs may receive untreated wastewater that has gone through preliminary treatment (e.g. screening and grit removal), or they may receive secondary effluent from some other treatment process, such as anaerobic reactors, activated sludge, or trickling filters. Typical concentrations for pathogens in wastewater, septage, latrine pit contents, and/or sludge are provided in Part Three of GWPP.
The outputs from WSP systems include the treated effluent (liquid), sludge/sediments (solids), and biogas. The treated liquid effluent from WSPs is often continuously discharged; however, operators of some systems (especially in colder climates) may stop discharging for months at a time, allowing the ponds to fill up and discharging once the temperature gets warmer (this extra retention time makes up for the slower rate of treatment during colder months). Sludge accumulates over time at the bottom of WSPs, and must be removed every few years (anaerobic ponds), every decade (primary facultative ponds), or every few decades (secondary facultative or maturation ponds). Sludge removed from WSPs is contaminated with pathogens and needs to be safely managed (to prevent exposure) or treated (to reduce the concentration of pathogens). Refer to the chapter on Sludge Management.
Figure 3. Typical inputs and outputs from waste stabilization pond systems
In WSPs, pathogens are removed from the wastewater or inactivated through a variety of different mechanisms that occur at different rates. The efficiency of these mechanisms is dependent on a number of environmental, design, and operational factors. The most important factors are shown in Figure 4. Systems that are poorly maintained, overloaded or malfunctioning may experience reduced pathogen removal efficiency, regardless of other factors. Likewise, WSPs with poor hydraulic efficiency (where the water entering the pond takes a preferential path short-cut directly to the exit point) will have poorer pathogen removal efficiency simply because the majority of pathogens do not stay long enough in the pond to experience the conditions that will remove or inactivate them.
Different factors affect different types of pathogens in different ways. The most important factor for the removal of viral and bacterial pathogens is sunlight exposure, although other factors such as temperature, dissolved oxygen and pH are also important. Sedimentation, hydraulic efficiency, sunlight exposure, and physical-chemical factors (including temperature and pH) are all important factors for the removal of protozoan pathogens, though sedimentation is perhaps the most important. Helminth eggs are primarily removed by sedimentation, and other factors are less important. Different pathogen types that are removed by the same mechanism are not necessarily removed at the same rate by that mechanism. For example, viruses and bacteria are both damaged by sunlight in WSPs, but viruses are generally more resistant than bacteria (Davies-Colley et al., 2005b; Sinton et al., 2002). Different species of viruses and bacteria are also removed at different rates in WSPs, due to differences in their structural and genetic composition (Silverman et al., 2013; Mattle et al., 2014; Kohn et al., 2016).
Figure 4. Major factors affecting pathogen removal in waste stabilization ponds and causal loop diagram showing the influence of sunlight on pathogen die-off
The UV portion of sunlight directly damages pathogen genomes (photobiological damage), while UV and visible wavelengths can react with photosensitizers in WSPs (such as natural organic matter) or photosensitizer molecules within bacteria (such as NADH/NADPH, flavins and porphyrins) to produce reactive species that indirectly damage pathogens (photo-oxidative damage). Sunlight is stronger at lower latitudes, higher elevations, and in locations with less cloud cover. The composition of WSP water is also important—sunlight is rapidly attenuated within the first few centimeters of WSPs (Davies-Colley et al., 2005a). Research has demonstrated a clear relationship between the amount of direct sunlight that reaches pathogens and their rate of inactivation (Nguyen et al., 2015; Silverman et al., 2015; Dias, 2016). Thus, the clarity of water in WSPs and the amount of sunlight penetration is a very important factor. Sunlight penetration can be improved in maturation ponds by designing them to be shallower than 1 m, a depth that is the current standard (maturation ponds can be as shallow as 20 – 40 cm (Maiga et al., 2009a; Jasper et al., 2013) as long as a liner is used to prevent the growth of emergent vegetation). This is especially true if the pond allows for the growth of periphyton (which grow at the bottom) instead of suspended-growth algae (which contribute to the turbidity of the water) (Jasper et al., 2013). Silverman et al. (2015) found no pathogen removal benefits to making maturation ponds any deeper than 50 cm.
Sunlight is one of the most important factors for viral and bacterial pathogen removal in WSPs. E. coli loses viability almost 20 times faster in WSPs with sunlight exposure compared to dark conditions, and it is also inactivated faster in shallower WSPs (Maiga et al., 2009a). Campylobacter jejuni is more vulnerable to sunlight damage than Salmonella enterica, which is more vulnerable than E. coli (Sinton et al., 2007). Enterococci and E. coli are equally as vulnerable to sunlight in clear water (Kadir and Nelson, 2014; Maraccini et al., 2016; Silverman et al., 2016), but enterococci were inactivated more rapidly than E. coli in a WSP in Ouagadougou, Burkina Faso (Maiga et al., 2009b), perhaps due to their greater vulnerability to sunlight in the presence of external photosensitizer molecules that may be present in WSPs with high concentrations of organic suspended solids (Kadir and Nelson, 2014). Bacteria can repair sunlight damage in the dark (Oguma et al., 2001), while viruses cannot (however, a few viruses may be able to repair sunlight-induced damage after initiating an infection, by utilizing their host cell’s DNA repair machinery (Weitzman et al., 2004).
Protozoan (oo)cysts are much more likely to be removed in a WSP system due to sedimentation than sunlight, but they are susceptible to sunlight damage—viable Cryptosporidium parvum oocysts were reduced by 0.2-log10 units in WSP water after four days of sunlight exposure (Reinoso and Bécares, 2008). C. parvum is capable of repairing sunlight damage, however it does not necessarily regain infectivity (Oguma et al., 2001). Regarding helminths, Ascaris eggs have high resistance to solar disinfection (Bandala et al., 2012; Heaselgrave and Kilvington, 2011). Thus, sunlight exposure is not likely an important factor for the removal of helminth eggs in WSPs.
WSP systems have hydraulic retention times on the order of days, weeks, or even months, which allows large, dense particles to settle. Sedimentation is more effective in WSPs with less turbulence. Ponds should be designed to maintain quiescent conditions that approach laminar flow. The size and density of pathogens and particles determines their settling velocities. Bacteria and viruses will not settle in WSPs unless they are attached to larger, denser particles. Only a small percentage of viruses attach to WSP particles, and they mostly attach to particles that are too small to settle (Characklis et al., 2005; da Silva et al., 2008; Krometis et al., 2009; Sobsey and Cooper, 1973; Symonds et al., 2014); this attachment is also reversible (Ohgaki et al., 1986; Sobsey and Cooper, 1973). Likewise, E. coli attaches to particles in WSPs, but this does not necessarily result in observable removal via settling (Boutilier et al., 2009). Relative to other factors, sedimentation is not very important for virus and bacteria removal in WSPs.
Protozoan (oo)cysts and helminth eggs are much larger than viruses and bacteria. The settling velocities of free-floating helminth eggs and protozoan (oo)cysts are 5 – 13 m/d and 0.026 – 0.13 m/d, respectively (compared to bacteria and virus settling velocities of 0.012 and 0.00001 m/d, respectively) (Cizek et al., 2008; David and Lindquist, 1982; Kulkarni et al., 2004; Medema et al., 1998; Sengupta et al., 2011). Cryptosporidium oocysts are among the smallest of the protozoan (oo)cysts and have low settling velocities, however they are hydrophobic and negatively-charged, which favors their attachment to particles (Characklis et al., 2005). Medema et al. (1998) found that 75% of C. parvum oocysts became attached to wastewater particles and settled at greater velocities as a result.
It is important to note that sedimentation does not necessarily result in the loss of pathogen viability, but rather transfers pathogens from the pond water to the sediment/sludge, where they may remain viable; flow velocities of 0.07 to 0.12 m s-1, high turbulence, and the forces generated by rising biogas bubbles can resuspend eggs and (oo)cysts from WSP sediments (Sengupta et al., 2012). Overturning resulting from diurnal or seasonal changes in temperature and the de-stratification of warmer and cooler water layers can also result in the resuspension of potentially viable pathogens in the sediment/sludge layer.
The most important physical-chemical factors for pathogen inactivation are pH, temperature and dissolved oxygen in the presence of dissolved organic matter. Most bacterial pathogens are vulnerable to high pH, with Vibrio spp. as a notable exception (Mezrioui et al., 1995). The sanitizing effect of free ammonia, which becomes more available at higher pH, is even more effective at higher temperatures (Decrey et al., 2014; Emmoth et al., 2011; Burge et al., 1983). Helminth eggs are the most resistant to physical-chemical factors in WSPs; they can survive for years in WSP sludge (Konaté et al., 2010; Nelson et al., 2004). High temperatures and the presence of volatile and organic acids, aldehydes, alcohols, and NH3 may increase helminth egg die-off in WSP sludge (Fidjeland et al., 2013; Ghiglietti et al., 1997; Nelson and Pecson, 2005; Nelson, 2003; Reimers et al., 1990).
Microbial diversity in WSPs presents situations that are stressful to pathogens. Bacterial pathogens (and viruses to a lesser extent) may be internalized by higher trophic organisms (Miki and Jacquet, 2008), however it is unclear if this affects their viability or transport through WSPs (Scheid and Schwarzenberger, 2012). Starvation affects fecal bacteria in WSPs, especially in maturation ponds with BOD5 concentrations < 20 mg/L (Almasi and Pescod, 1996). Algae and fungi also excrete metabolites that are harmful to some viruses, bacteria, and protists (Metting and Pyne, 1986; Mille-Lindblom and Tranvik, 2003; Senhorinho et al., 2015).
As illustrated in the previous section, the performance of a WSP system depends on a multitude of factors interacting simultaneously, and frequently it is not possible to identify a single factor that explains poor or good performance (Oliveira and von Sperling, 2011).
Nevertheless, there are several design approaches that can help enhance pathogen removal in WSP systems. Systems with longer hydraulic retention times and a greater number of ponds in series are associated with greater removal of helminth eggs (Ayres et al., 1992; Saqqar and Pescod, 1992), protozoan (oo)cysts (Ben Ayed et al., 2009; Reinoso et al., 2008), bacteria (von Sperling, 2005), and viruses (Verbyla and Mihelcic, 2015). The theoretical hydraulic retention time (calculated as the pond volume divided by the flow rate) however, is not the only factor that affects the residence time distribution (the amount of time pathogens actually spend in the system) (Persson, 2000; Shilton and Harrison, 2003a). Other design factors such as the pond depth, the length-to-width ratio, the configurations of inlets and outlets, the speed and direction of the wind, stratification caused by diurnal shifts in the temperature at the surface of the pond, and the presence or absence of baffles can all have an impact on pathogen removal, particularly in maturation ponds. Maturation ponds with higher length-to-width ratios (longer ponds or ponds with baffles) are generally more efficient at reducing pathogens (von Sperling, 2007). The strategic design and positioning of inlet and outlet structures and stub baffles in WSPs can improve hydraulic efficiency and enhance pathogen removal (for more, refer to Shilton and Harrison (2003a, 2003b). Pond depth is also an important factor, particularly for maturation ponds. Anaerobic and facultative ponds are designed to remove organic matter and must be deeper by necessity, but shallower maturation ponds experience better sunlight penetration and increased photosynthetic activity (leading to higher pH and dissolved oxygen), which improves pathogen removal. Pilot studies with extremely shallow maturation ponds (20 cm), installed with a liner to prevent emergent plants that would otherwise shade the water surface, have demonstrated the development of periphyton communities and more efficient pathogen removal than conventional maturation ponds (Nguyen et al., 2015; Silverman et al., 2015). Shallow ponds with full sunlight exposure also experience higher temperatures than deeper ponds, which can enhance pathogen removal (Maiga et al., 2009a).
Proper operation and maintenance are crucial to achieving efficient pathogen removal in WSP systems. Improper maintenance can lead to malfunctions that significantly reduce pathogen removal (Verbyla et al., 2016). Due to their large volumes and long hydraulic retention times, WSPs are usually robust to short-term (daily) fluctuations in the influent, related to flow, concentrations or loads. However, long-term malfunction concerns, such as sludge accumulation, hydraulic short-circuiting, and organic or hydraulic overloading, may reduce the efficiency of pathogen removal. Organic and hydraulic overloading may be especially problematic in cities and towns with rapid population growth or increasing industrial activity. WSPs that are organically overloaded may turn anaerobic, becoming more turbid and allowing less sunlight penetration. The clarity of the water in WSPs is indeed a very important factor for pathogen removal in WSP systems (Davies-Colley et al., 2005; Silverman et al., 2015). Hydraulic overloading can cause hydraulic short-circuiting which reduces the effective time pathogens remain in the system (Verbyla et al., 2013a). Sediments (sludge) will accumulate at the bottom of WSPs (especially anaerobic and facultative ponds) over time, diminishing the effective pond volume, potentially causing overturning and the resuspension of settled pathogens, reducing overall pathogen removal (Verbyla et al., 2016). Modeling has demonstrated that sludge accumulation in ponds can sometimes act as a baffle, potentially improving hydraulic efficiency (Ouedraogo et al., 2016). Nevertheless, sludge should be removed every few years for anaerobic ponds, at least every 10 – 20 years for primary facultative ponds, and every few decades for secondary facultative and maturation ponds. WSP systems should be designed with ponds in parallel to facilitate the process of sludge removal.
The removal of pathogens in WSPs clearly depends on a number of environmental factors as well as engineering design and operation and maintenance practices, as demonstrated in the previous section. Data from a review of the literature on pathogen removal in WSPs were collected; if available, data on the following three factors were also collected: 1) theoretical hydraulic retention time, 2) average air or water temperature, and 3) pond depth. The effect of hydraulic retention time on pathogen removal is shown in Figure 5 and the effects of temperature and pond depth on pathogen removal are shown in Figure 6.
Hydraulic retention time certainly influences pathogen removal in WSPs, but it is not the only factor. In general, for facultative and maturation ponds with HRTs up to 80 days, the log10 reduction of coliforms is generally greater than that of viruses (with a few exceptions). The removal of helminth eggs is generally similar to or greater than that of coliforms (Figure 5). There are few studies on the reduction of protozoan parasites (protists) and bacterial pathogens in WSPs, but based on the data available, it appears that their reduction may be similar to that of coliforms.
Figure 5. Relationship between the theoretical hydraulic retention time (for individual ponds) and the log10 removal of pathogens and coliforms (including E. coli) in facultative and maturation ponds. Data for pathogens are shown together with data for traditional fecal indicator bacteria for comparative purposes
A simple way to normalize pathogen removal efficiency in WSPs with different hydraulic retention times is to calculate a ratio of the theoretical hydraulic retention time (HRT) divided by the log10 removal. This ratio, which represents the theoretical time required for each log10 reduction, may be used as a very simple proxy for planning and design purposes. However, this approach should be used with caution, as the data presented in Figures 5 and 6 have variations that span orders of magnitude. When using this approach, also consider that the data presented in Figure 6 come from studies that found at least 1log10 reduction, but generally no more than 2log10 reduction of pathogens. Figure 6 shows the relationship between ambient air temperature, pond depth, and the time required for each log10 reduction (note that the data shown in Figure 6 come from studies of ponds that provide between 1 and 2log10 reduction of pathogens). As a general rule, increasing the number of ponds in series should improve hydraulic efficiency and pathogen removal efficiency. For example, one pond with a retention time of 20 days will probably not remove pathogens as well as three ponds in series with an overall retention time of 20 days.
Also, deeper (facultative) ponds generally require more retention time to achieve the same level of pathogen removal as shallower (maturation) ponds, as indicated by Figure 6. There are extremely large variations in the number of days per log10 reduction. This is due to the fact that depth and temperature are only two of many other factors that are also important for pathogen removal in WSPs (e.g., Table 1).
Figure 6. Relationship between temperature, pond depth, and the number of days per unit log10 reduction for waste stabilization ponds achieving at least 1log10 removal of pathogens and fecal indicators. Temperatures shown represent average air temperature (for pathogens) and average water temperature (for fecal coliforms)
Over time, some of the pathogens in the wastewater entering WSPs will end up in the sediments that accumulate at the bottom of the pond. Sedimentation does not necessarily result in the inactivation of pathogens, which may remain viable in WSP sludge/sediments (Table 3). The pathogens of greatest concern in WSP sludge, due to their durability and persistence, are helminths and protozoans. Concentrations of helminths reported in WSP sludge generally do not exceed 1,000 eggs per g total solids (TS), and in many cases are less than 100 eggs per g TS (Amahmid et al., 2002; Konaté et al., 2013a, 2010; Nelson et al., 2004; Schwartzbrod et al., 1989, 1987; Verbyla et al., 2013b), however von Sperling et al. (2003) found values close to 1,000 eggs per g TS, Oakley (2004) found more than 4,000 eggs per g TS of WSP sludge from one system in Honduras (which also had relatively higher concentrations of helminth eggs in the raw wastewater entering the system). The concentrations of helminth eggs in WSP sludge likely depend on the incidence of helminth infections in the community. Helminth eggs in particular have been shown to persist and maintain viability for long periods of time in WSP sediments. For example, protozoan cysts and viable helminth eggs were detected at high concentrations even at the bottom layer of WSP sediments in an anaerobic pond that was four years old (Konaté et al., 2013a, 2010); and viable helminth eggs were detected in a WSP sediment layer that was estimated to be 12 years old (Nelson et al., 2004). Konaté et al. (2010, 2013a) reported the following percent viability of helminth eggs in WSP sediments: 36 – 51% (A. duodenale), 40 – 100% (Ascaris), 22% (Dicrocoellum spp.), 14% (E. vermicularis), 21% (H. nana and N. americanus), 6 – 13% (T. trichiura). Table 3 provides a summary of typical pathogen concentrations in WSP sediments.
Sediments and sludge removed from WSPs are highly concentrated with potentially viable pathogens, particularly helminth eggs and protozoan (oo)cysts, and therefore, must be managed or treated appropriately to avoid exposure or reduce the concentration of viable pathogens. The recommended treatment method for sludge excavated from WSP sediments is either burial in a nearby field (to limit human contact), or dewatering in an open field or a sludge drying bed, where the material should be stored in the sunlight and out of the rain for a minimum of one year prior to reuse. Engineered systems such as greenhouse solar drying beds may help speed up the process by desiccating the material and raising temperatures during the day. The addition of seeds from wetland plants such as Ludwigia spp. can also help accelerate the process of dewatering (Oakley et al., 2012). Lime can be added to sludge removed from WSPs to stabilize the sludge and increase the pH to inactivate pathogens.
Tables 2 and 3 provide summaries of typical pathogen removal efficiencies in WSPs and presence of pathogens in the pond sediments (sludge). The WSPs used to generate the data in these tables had hydraulic retention times of <10 days (for anaerobic ponds) and <30 days (for facultative and maturation ponds). Therefore, a design engineer sizing anaerobic ponds with 10 days of hydraulic retention and facultative or maturation ponds with 30 days of hydraulic retention can likely anticipate the pathogen removal efficiencies shown in Table 2. Concentrations of pathogens in WSP sludge should not greatly exceed the high end of the ranges reported in Table 3, however concentrations may be lower depending on the prevalence of infection within the community. In general, the compilation of results from the literature (summarized in Figures 5 and 6 and in Table 2) indicates that the efficiency of pathogen removal in full-scale WSP systems can be highly variable, and depends on pond depth and temperature, but also on a number of other factors (e.g., Table 1). Hydraulic efficiency and sunlight penetration are two factors that appear to be very important for maximizing pathogen removal efficiency in WSPs, yet they have not been very well-studied. Community acceptance is another important (non-technological) issue for waste stabilization ponds, as some communities view ponds as outdated technologies, despite their ability to removal pathogens, with negligible energy and material inputs, just as well as many mechanized sanitation technologies.