Elsevier

Ecological Engineering

Volume 50, January 2013, Pages 62-68
Ecological Engineering

Greywater treatment by slanted soil system

https://doi.org/10.1016/j.ecoleng.2012.04.028Get rights and content

Abstract

Slanted soil system is a low cost, simple system suitable for greywater treatment. Performance of the slanted soil system for greywater was evaluated in light of required water quality for irrigation reuse. Removal rate of particle COD and BDOC, those may cause clogging of irrigation facility, were 94–97% and 88–89%, respectively. LAS removal rates were more than 90% and final concentrations (2.3–3.3 mg L−1) were sufficiently lower than proposed target level for irrigation use (8 mg L−1). Only fine soil (1–4 mm) performed 5 log10 and 3 log10 reductions of Escherichia coli and MS2 phage, while coarse soil could not remove those pathogens. Clogging was observed in fine soil after 3–5 weeks operation, however combination of coarse soil chamber and fine soil chamber could extend it to 8 weeks. Reductions of total COD and LAS were described by 1st order reaction model and reaction coefficient k was described by equation of per area discharged rate.

Introduction

Naked dry soil with no plant is difficult to keep rich soil ecosystem and it can be easily lost by wind or rain erosion. This kind of soil deterioration has considerable effect on ecosystem and agricultural productivity in arid or semi-arid zone (Lal, 1998, Visser et al., 2005). Planting through a year can reduce this soil deterioration; however, water limitation does not allow it in these areas. For example, in rural area of Burkina Faso, which belongs to semi-arid zone, farmers can cultivate their main farmland only during rainy season. Only small garden which located in shoreline of reservoir or near the well, are able to be cultivated in dry season (Ushijima et al., 2012). Distribution and time periods of available water greatly affect the agricultural activity in this area, and most farmers are facing poverty problem due to such low productivity. On the other hand, people in this area use some amount of water for daily life even in dry season, and its wastewater is just disposed (Ushijima et al., 2012). Effective wastewater reuse has a potential to increase the cultivatable area and season, and it provides not only reduction of soil deterioration but also increase of productivity.

The concept of onsite wastewater differentiable treatment system (Lopez Zavala et al., 2002) can be one of suitable system for above-mentioned situation. It proposes onsite treatment of greywater, urine and feces separately, in order to achieve effective resource recycling system. Separated greywater has low concentration of pollutant load (Gajurel et al., 2003) and its treatment would be easier than mixed wastewater. For example, WHO (2006) wastewater reuse guideline stated that 2–7 log10 pathogen reduction by treatment is required for irrigation use, however separated greywater contains 2 log10 or more lower bacterial pathogen than mixed wastewater which WHO assumed (Ottoson and Stenström, 2003) and therefore we are able to set 5 log10 as a target level of bacterial pathogen reduction for separated greywater treatment. Furthermore, if reuse purpose was focused on irrigation only, advanced treatment technology for nitrogen, phosphorus removal would not be necessary. The most important point is to be low cost and simple, because most of those who need greywater reuse are low income people.

The slanted soil system is one of the promising treatment systems for this concept. It consists of several chambers containing soil (Fig. 1). These chambers are able to be stacked vertically, therefore its footprint is very small (e.g. approximately 1.0 m × 0.5 m). This system accepts direct discharge of greywater (Kondo et al., 2011) therefore no pump and no septic tank are required. Itayama et al. (2006) and Kondo et al. (2011) performed continuous monitoring of slanted soil system used by real household in Japan. Itayama et al. (2006) reported averaged removal ratio of the COD, the SS, the TN and the TP as 85%, 78%, 78% and 86%. Li et al. (2009) referred this result and concluded that the treated water is not suitable for reuse because it remains high in organic load and suspended solids, which can limit the chemical disinfection. These are however not the discussion for irrigation use but for avoiding eutrophication (Itayama et al., 2006) or for potable reuse (Li et al., 2009). Therefore, in this study, we evaluated the performance of the slanted soil system as a treatment facility for irrigation reuse, and proposed its design criteria for arid or semi-arid zone.

Section snippets

Experimental apparatus and operation

Full scale slanted soil system was installed inside of laboratory; four soil chambers were connected as shown in Fig. 2, the size of each chamber was 0.10 m of depth, 0.145 m of width, and 0.94 m of length, chamber bed has gradient of 1/20. We performed 6 cases of experiments, in variety of soil type, soil particle size and amount of discharged wastewater (Table 1). In all experiments, greywater was discharged to first chamber 3 times in a day because actual discharge pattern of greywater shows 2

SS removal and clogging

SS removal by treatment length was almost linear in logarithmic graph (Fig. 3). Total removal rates through 4 chambers were 60–94%, and these were higher in finer particle and lower discharge (Fig. 4). Discharged water was overflowed from the chamber due to clogging in Cases 3, 4, 5 and 6 at the 5th, 3rd, 8th and 3rd week, respectively. Time periods until clogging (T) were longer in coarser soil and higher discharge. In Case 5, combination of fine and coarse soil, T was longer than those using

Treatment performance

Fine soil presented better removal performance of pathogen, SS, and T-COD. Particularly, sufficient pathogen removals were performed only by fine soil chamber, while most part of pathogen passed through coarse soil chamber. Disadvantage of fine soil was shorter time periods until clog, however result of Case 5 represented that combination of coarse and fine soil can extend this time period to more than double. Most of previous studies used Kanuma soil, which is generally 10 mm or larger particle

Conclusions

Performance of the slanted soil system for greywater was evaluated in light of required water quality for irrigation reuse. The slanted soil system performed high removal rate in both P-COD (94–97%) and BDOC (88–89%), while removal rate of D-COD (58–68%) was comparatively low. LAS removal rates were more than 90% and final concentrations (2.3–3.3 mg L−1) were sufficiently lower than 8 mg L−1 which was proposed as target level for irrigation use (Hijikata et al., 2011). First order reaction equation

Acknowledgements

This research was supported by JST-CREST, JST-JICA and JSPS-Science Research (type S).

References (20)

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