Gray Water Characteristics and Implicatiobs on Regulations for Sustainable Water Reuse

Different types of gray water in terms of origin (from bathing, kitchen and laundry activities) and in terms of type of source (medium and low-income residential housing and a hotel) have been studied in Gaborone, Botswana during 2004, with respect to BOD₅, TSS, TP, pH, fecal coliforms and conductivity. Results show an expected high variability. No statistically significant difference was found with respect to the different types of sources, however, the low-income houses’ samples showed higher maximum concentrations compared to the medium-income ones, except for pH. The mean values of the laundry gray water from a hotel were significantly higher with respect to pH (~ 9) and conductivity (700 - 9000 µS/cm). Kitchen gray water samples showed a pH lower by 1 unit compared to the bathing and laundry samples from residences (pH = 7.0 - 7.3). TP values showed concentrations 10 - 50mg/l for laundry and kitchen samples. Implications in respect to sustainable types of beneficial reuse and corresponding treatment technology have been discussed. Specific aspects of the regulatory basis for the practical implementation of this practice have been presented with emphasis on the need for the introduction of the decentralized wastewater system approach, parallel to the existing practice of centralized systems.

Content Table

• Introduction
• Methodology
• Results and Discussion
  • Spatial gray water quality variation
  • Treatment options
  • Regulatory aspects
• Conclusions
• References
• Related Articles

Introduction

Internationally, the vast majority of water reuse cases apply to centralized wastewater systems, where water is collected from the whole population center and is treated in relatively large treatment facilities, which produce a bulk wastewater volume on a continuous and uninterrupted basis. Its beneficial reuse would require complex design and operation structures, and in the case of irrigation – large irrigation fields, specific irrigation timetable, water transportation and storage facilities.  A relatively new development in the wastewater reuse practice is the introduction of the decentralized wastewater collection and treatment systems, which apply a localized (in many cases “on-site”) treatment of wastewater and its beneficial reuse. The level of decentralization could vary, and this approach could be applied to individual households, individual blocks of flats or small complexes, institutional organizations, educational facilities, recreational/holyday complexes, etc. In cases, where treated water could be reused for irrigation of gardens and small-scale agricultural activities, washing of courts, etc, the collection, treatment and reuse of municipal wastewater is a relatively non-expensive and easy to operate and maintain practice. Within the concept of decentralized wastewater systems, a possible alternative is the separation of two distinct flow lines in terms of collection, treatment and reuse, named “black” and “gray” water. “Black water” is the wastewater generated by the use of toilets, while “gray” water is the wastewater generated from washing, bathing and cooking activities. This differentiation is based on the distinction in the wastewater characteristics of both flows, which would require a different approach to the level of treatment, corresponding treatment technologies and appropriate reuse options. It should be emphasized, that the introduction of the decentralized approach in terms of practical implementation should be considered as a viable option for new urban developments, and is most suitable in cases, where a rapid urban population growth leads to high population density and overloading of the existing sewer system.

This article aims to present a quality characterization of gray water, specific for developing countries in arid regions. Also, it discusses the implications of gray water characteristics with respect to the regulatory approaches, which need to be developed for the implementation of such practice, and appropriate treatment technologies for small-scale applications in individual households or institutional / commercial enterprises.

Methodology

Characteristics of gray water have been determined in Gaborone, the capital city of Botswana during the period June – December 2004. Differentiation has been made among different types of gray water, based on the origin: gray water from bathing activities (index B), from kitchen sink (index K) and from laundry (index L). Also, differentiation has been made based on the social habits and availability of water, thus samples have been collected from three locations – one household in a medium density residential area (H1), one household in a high-density residential area with poor infrastructure (H2), and at a small hotel (F) in Gaborone, Botswana. H1 houses could be regarded as typical for medium income family, developed on plots varying between 300-500 m², provided with reticulated water supply and a centralized sewer system. H2 houses could be regarded as typical for low-income families, where water is supplied at selected locations in the neighborhood and sanitation facilities consist of pit latrines. The F sampling location corresponds to a three star hotel. A schematic representation of the different sampling locations, as well as pictures of two sampling sites are shown in Figure 1.

Samples have been collected on four occasions for locations H1 and H2 and five occasions for location F.  Composite samples have been taken for a period of 2 hours from both sampling locations at F location. At H1 location, B samples have been collected as composite samples of one bathing procedure, K samples are the composite of manual dishwashing procedures, and L samples are the composite of one laundry procedure, executed by an automatic washing machine. At H2 location, the K and L samples are collected as composite samples of a manual dishwashing and laundry process in a basin, including one basic washing procedure and one rinsing procedure. Field measurements and laboratory tests were performed with respect to BOD, total suspended solids (TSS), conductivity, pH, temperature, fecal coliforms (FC), and total phosphorous (TP) according the Standard Methods (1992). TP has been determined by the Vanadomolybdophosphoric acid colorimetric method after nitric acid-sulfuric acid digestion, the FC were determined by the membrane filtration method, and BOD as BOD₅. Statistical data analysis of the data sets has been performed, and comparison for statistically significant difference between the different data sets has been done based on EXCEL standard statistical tools, at 95% confidence interval. Throughout the text, the term significant difference applies for statistically significant mean values of the data sets.

Results and Discussion

Considering the nature of formation of gray water, it could be expected that its characteristics would vary in a wide range, based on the different household activities applied, the type of laundry and kitchen detergents including the whole range of products used in routine domestic activities, as well as, the common housekeeping habits and the availability of water. Such wide variation has been confirmed during this study (Table 1, and Figures 1,2), as well as in other investigations (Hoko, 1999; Jefferson et al. 2001; Patterson 2001).  Considering this fact, the observed mean concentrations should be regarded as orientation values, which show a range in which the different constituents could vary, and should be interpreted in terms of the standard variation (SD) of the data set.

a)      Schematic representation of the sampling locations and coding

b) H1 type of housing and sampling location HK1

c) Washing dishes at H2 type of housing development

Figure 1. Sampling locations

Figure 2. Gray water quality variation – laundry activities (numerical values denominate SD of the data set)

Figure 3. Gray water quality variation – kitchen activities (numerical values denominate SD of the data set)

Table 1. Gray water quality variation – bathing activities

Observations in respect to temperature were done on the field during individual sampling procedures and show a variation between 20ºC and 40ºC. Mean temperature values were not determined because samples were collected as composite ones. FC tests showed a very high variation, with numerous observations of zero counts, but on selected occasions, especially from bathing and laundry activities, values between 1000 to 10 000 cfu/100ml were detected. This shows that there is a public health risk, if such water is to be reused directly (without disinfection) in cases requiring direct human contact with the recycled water, but such hazard is minimum in cases of reuse in restricted accesses areas (Anderson et al. 2001).

The range of variation of the tested parameters (Figures 2 and 3, Table 1) with respect to the different types of gray water is within limits reported by other investigations (Hoko, 1999; Jefferson et al. 2001; Patterson 2001, Draft guidelines, 2002). The only parameter, which showed relatively low SD, was pH.

Spatial gray water quality variation

Comparison of the mean values of the parameters tested with respect to different types of gray water and the different types of sources of gray water was done in order to detect statistically significant values. Results, regarding significant differences are presented in Table 2. The relatively low number of significant differences is due to the high variability of the data sets. In general, the different types of gray water did not show significant difference in respect to organic constituents (BOD), except for the hotel, where the kitchen samples were significantly higher compared to laundry samples. The same applies for TSS, where bathing samples had significantly lower values compared to laundry samples (HB/FL). However, the range of TSS and BOD variation of bathing samples was considerably lower, compared to the other two groups, with the highest concentrations detected in HL2, HK2 and FK. With respect to TP, gray water from bathing activities had significantly lower concentrations compared to kitchen and laundry. The maximum TP concentrations (about 50mg/l) were detected in laundry samples (HL2 and FL) and are comparable to the ones reported by Patterson (2001). The parameter with the lowest SD - pH did not show significant difference between bathing and laundry values in domestic conditions (HB, HL1, HL2) with a range between 7 and 7.5, but in general, the kitchen samples were lower by one unit, and the laundry samples from the hotel (FL) were higher by one unit. Dissolved solids represented by conductivity, showed significantly higher mean values with respect to laundry samples. The highest concentrations (up to 9000 µS/cm) were observed regarding FL, which explains and the increased pH values, but significant difference was not proved due to high SD.

Table 2. Positive results of the test for significant difference of mean values

Treatment options

The choice of suitable treatment options should be made based on the selection of an appropriate type of beneficial use. In general, with respect to all cases of wastewater reuse, the treatment technology applied should be designed and operated in conjunction with the reuse structure and should be treated as one system.

In many cases of practical implementation of centralized wastewater systems and reuse options, emphasis is given to the wastewater treatment technology, with little or limited attention to the subsequent structure, which uses the reclaimed water. In cases of reclaimed wastewater used for irrigation in developing countries, often the treatment plant operates independently of the irrigation field, leading to hydraulic and contaminants overloading of the irrigated site, with consequent pollution of soils and ground water (Hranova 2002). A systematic approach to the wastewater reuse practice would require a sound planning procedure based on preliminary survey and collection of actual data in respect to both – the treatment process and the reuse option. Failure to do so might result in unsustainable solutions in terms of economic considerations, as well as in terms of environmental and public health protection. Hermanowicz et al. (2001) reported an example of a centralized wastewater reuse project, which proved to be economically unsustainable, because of overestimation of the demand for reused water. In other cases, the design of wastewater treatment facilities, based on literature data, lead to considerable overestimation of the available quantities of reclaimed water for irrigation purposes and resulted in an additional unplanned demand of fresh water in order to sustain the function of the reuse structure (Hranova, in preparation).

Irrigation of agricultural fields, gardens, parks and golf courses is by far the most widely reuse alternative since ancient times. It is a viable and sustainable option, especially for developing countries, because of the relatively low cost required for the reclamation of wastewater and the fact that it could be applied at all scales – large irrigation fields or household gardens. In arid climates, the possibility for rain fed agriculture or gardening is limited, thus the reuse of wastewater could save a considerable amount of fresh water. At household and institutional level, the reclaimed gray water would actually save water of potable quality used for irrigation of loans and gardens, and would contribute to an aesthetic environment. Other reuse options and corresponding treatment requirements could be applied as well, based on a sound preliminary survey and at an affordable cost. Different wastewater reuse alternatives and corresponding treatment techniques could be recommended for small-scale reuse in medium-density residential developments, blocks of flats, and institutional/commercial enterprises as shown on Figure 4. The provision of combined storage of all types of wastewater would allow for mixing and equalization of the total reclaimed volume and would serve as a sedimentation volume, considering the substantial TSS concentrations. It would also provide the possibility to control the flow to subsequent treatment units and reuse options. The determination of the storage volume could be based on a 24-hours collection of all gray water flows and the regime of subsequent treatment and reuse.

Figure 4. Recommended gray water reuse options and corresponding treatment techniques

A point of concern with respect to gray water reuse in hotels and other commercial enterprises releasing laundry gray water is the high dissolved solids concentrations. It would require a special consideration, where different possibilities could be envisaged, such as more economical use of detergents, application of special laundry products, or a separate treatment of this type of gray water in cases of larger projects.

Regulatory aspects

The challenges for the regulation of the gray water reuse practice are significant, considering that the implementation of such practice would require a basic change in the existing regulations and plumbing codes. One of the most important aspects to be considered in this direction is the level of centralization of the existing and new wastewater treatment systems and the way forward. The trend is to give emphasis on centralized systems for wastewater reuse, where a bulk volume of reclaimed water is produced and distributed to one or different consumers. If such strategy is to be adopted, it would contradict directly a more decentralized wastewater reuse practice by private entities on a larger scale, because would lead to a considerable decrease in the planned quantities available for centralized reuse. Such is the case of the City of Windhoek in Namibia, where the vast majority of the municipal wastewater is reused for potable purposes and the municipal authorities are reluctant to allow a gray water reuse practice (City of Windhoek, 2004). In addition, gray water forms about 40-60% of the total amount of domestic wastewater and a massive practice of gray water reuse within a sewered area (gravity sewers) would decrease significantly the actual sewer flow. This would result in solid depositions, clogging and other operational problems. Therefore, the decision what approach to adopt should be made after a careful consideration of the specific circumstances and the levels of centralization. The gray water reuse in a typical decentralized approach for a new suburb would require the consideration not only of gray water but black water treatment and reuse options as well, and also alternative solutions for the sewer system (pressurized or vacuum sewers), or a completely individual on-site sanitation technologies. Other possible alternative is the combined use of reclaimed wastewater and storm water. Thus, the practical implementation of gray water reuse would rely heavily on the vision, competence and willingness of local authorities to adopt a new and unconventional strategy and to develop comprehensive guidelines and regulations. The decision making process in this direction could be supported by the development of risk assessment models and optimization techniques (Luke, 2001; Diaper et al. 2001).

The development of a code of practice or guidelines for gray water reuse would help to avoid public health and environmental risks. An important aspect is the level of restriction to be included. A decentralized approach to wastewater reuse would envisage the consumers to take the initiative and responsibility for the implementation of such practice and the maintenance of the facilities. Therefore, the regulatory documents should be enough specific to provide for a save practice in the form of guidelines, considering the local conditions. In addition, restrictions to prevent pollution of neighboring premises and the environment should be included. The preparation of such guidelines should be based on sound information in respect to the quality and quantity of the wastewater to be reused and environmental factors, such as types of soils, ground water levels, rainfall data, etc. Another important aspect to be considered during the development of regulatory instruments is their flexibility, which should reflect actual habits and practices and would incorporate them in the existing regulatory structure, allowing for a wide range of choices. For example, the recommended gray water treatment and reuse options (Fig. 4) are not practical for the cases of low-income residential areas, which are similar to numerous cases of traditional African lifestyle in rural areas, except that urban developments are very overcrowded and located close to each other. For such conditions, the guidelines could provide for a direct reuse of the gray water for irrigation of bushes and trees on rotational principle, backed up by educational programs for save sanitation and gray water reuse practice.

Decentralized wastewater systems in general and gray water reuse systems in particular could not be subject to the conventional monitoring and control practice, applied with centralized systems. The numerous individual cases could not be controlled and monitored by a central authority, but should be supervised and operated by the owners. However, the application of such type of installations should be subject to a permit, similar to permits for water connection. Also, the local authority could provide services to the owners in the form of specialist consultations and/or provision for maintenance. The implementation of such practice would require a broad basis of public awareness and education in respect to the possible health risks, existing regulatory documents, as well as, penalties for violation of the prescribed restrictions. Therefore, the planning stage of the implementation of such practice should include a specific program orientated towards public awareness and education. As part of such programs, the implementation of demonstration projects (Hermanowicz et al. 2001) could be a viable option for detailed costs estimation and promotion to the public. 

Conclusions

Gray water characteristics at selected locations in Gaborone, Botswana show a typical high variation in the observed concentrations, but are within the range of reported values. With respect to different types of housing development no significant difference of the observed parameters was found, however, the H2 samples showed higher maximum concentrations compared to H1 samples, except for pH. The mean values of the laundry gray water from a hotel were significantly higher with respect to pH (about 9) and conductivity (within a range of 700 to 9000 (µS/cm). Kitchen gray water samples showed a pH lower by 1 unit compared to the bathing and laundry samples from residences. The observed TP values showed concentrations in arrange of 10 to 50mg/l for laundry and kitchen samples.

Based on the results obtained and different alternatives for reuse options, treatment techniques were recommended, which could be considered during the process of development of guidelines for gray water reuse. With respect to a sound regulatory basis for the implementation of gray water reuse, emphasis was given to the need to develop such documents as part of a strategy for decentralized wastewater treatment, which should complement the existing practice of centralized wastewater treatment and reuse. 

Acknowledgements: Thanks for the financial support provided by research grant 560 from the University of Botswana Research Foundation, and to all support staff, which took participation in the project.

References

This article was written by:

R. Hranova

Department of Civil Engineering, University of Botswana, P. Bag 0061, Gaborone, Botswana, hranova@mopipi.ub.bw

The following references were used:

Anderson J., Adin A., Crook J., Davis C., Hultquist R., Jimenez-Cisneros B., Kennedy W., Sheikh B., van der Merwe B. (2001).Climbing the ladder: a step by step approach to international guidelines for water recycling Wat. Sci. Tech., 10 (43), 1-8.

City of Windhoek (2004) Sewerage and Drainage Regulations (amendments) Local authorities act, section 23, 1992, Windhoek,Namibia

Diaper C., Dixon A., Butler D., Fewkes A., Parsons S.A., Strathern M., Stephenson T., Strutt J. (2001). Small scale water recycling systems – risk assessment and modeling Wat. Sci. Tech., 10 (43) 83-90

Draft guidelines for the reuse of gray water in Western Australia (2002) http//:www.health.wa.gov.au

Hermanowicz S.W., Sanchez Diaz E., Coe J. (2001) Prospects, problems and pitfalls of urban water reuse: a case study Wat. Sci.Tech., 10 (43), 9-16

Hoko Z. (1999). Innovative sanitation for Harare, Zimbabwe (case study Mufakose suburb). MSc thesis SEE 080, IHE Delft, The Netherlands

Hranova R. K. (2002). Water Reuse in Zimbabwe – an Overview of Present Practice and Future Trends In: Proceedings of 3^rd International Conference on Integrated Environmental Management in Southern Africa, August 27-30, Johannesburg,RSA.

Hranova R. (in preparation) The wastewater reuse practice in Botswana – a challenge for the development of the water sector. In:Proceedings of BIE 2005 Annual Conference, 19-21^st October, Gaborone, Botswana.

Jefferson B., Laine A.L., Stephenson T., Judd S. J. (2001). Advanced biological unit processes for domestic water recycling Wat.Sci. Tech., 10 (43), 211- 218

Lucke F.K. (2001). Assessment of the risk from use of water from grey water recycling systems In: Proceedings of the 10th International Conference on Rhine- water catchment systems, September 10-14, Mannheim, Germany

Patterson R.A. (2001). Wastewater quality relationships with reuse options Wat. Sci. Tech., 10 (43), 147-154 Standard Methods for the Examination of Water and Wastewater (1992). 18th edn, American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC, USA.

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