Comparison of Irrigation Qualities of Septic Tank Effluents Reclaimed Using Aerobic- versus Anaerobic-Based Treatment Systems

www.sciencetarget.com Comparison of Irrigation Qualities of Septic Tank Effluents Reclaimed Using Aerobic- versus Anaerobic-Based Treatment Systems International Journal of Environment and Sustainability [IJES] ISSN 1927-9566 Vol. 5 No. 1, pp. 64-70 (2016) *Correspondence: aabusam @kisr.edu.kw A. Abusam *, M.I. Ahmed, and A. Mydlarczyk Wastewater Treatment and Reclamation Technologies (WTRT) Program, Water Research Center Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait. Abstract. Conventional septic tanks remove partially suspended solids and organics from wastewater, but they do not remove nutrients and pathogens. The Kuwait Institute for Scientific Research (KISR) studied a number of promising systems for the reclamation of effluents from conventional septic tanks. This paper compares the irrigation qualities of the final products of aerobic- versus anaerobic-based post-treatment systems. Reclamation of the effluents of a fullscale conventional septic tank indicated that an anaerobic-based treatment system resulted in lower salinity, fewer suspended solids, and smaller amounts of organics than an aerobic-based treatment system. However, the aerobicbased treatment system was found to be more efficient in the removal of total nitrogen. Obtained results have also indicated that both treatment systems give almost the same concentrations of boron and heavy metals. Therefore, it has been concluded that reclamation of effluents of conventional septic tanks requires both aerobic and anaerobic biological treatments Keywords. wastewater, post-treatment, septic tank effluents, irrigation qualities, aerobic treatment, anaerobic treatment 1. Introduction Wastewater of individual households or collection of households is often treated in a septic tank system, which is the oldest wastewater treatment system (Okereke, 1997). In many on-site wastewater treatment situations, the septic tank system is considered to be the best option and disposal, as it is relatively inexpensive and almost a maintenance-free system. It has no moving parts and it does not require chemical addition (Fox, 1984). For these reasons, septic tank systems are the most commonly used on-site treatment and disposal systems all over the world. About 25% of the homes in the United States of America (USA) use septic tanks. Also, about 22 million people in Europe and about 60% of the Chinese population depend on the septic tank system (US Census Bureau, 1999). The septic tank system is also the common onsite wastewater treatment system in remote, non-sewer areas of Kuwait. A conventional septic tank system usually consists of an underground watertight tank and a drain field, which usually consists of trenches filled with gravel and sand. The septic tank keeps the wastewater for some period of time (preferably greater than 24 h) in order to be treated anaerobically. The main functions of the septic tank are: (i) to partially digest the organic matter, (ii) to remove settleable and floatable materials and accumulate them as sludge and scum, and (iii) to store the sludge until tank cleaning (Viraragahavan, 1976). On the other hand, the main function of the drain field is to allow further treatment of the septic tank effluents before percolation into the surrounding soil. Thus, conventional septic tanks remove only parts of the suspended solids and organics through sedimentation and anaerobic digestion, before discharge into the soil. Thus, they

International Journal of Environment and Sustainability, 2016, 5(1): 64-70 65 provide basically primary treatment of wastewater. Further, they are inefficient in the removal of solids and organics due to the fluctuations in wastewater flow rates and gasification that often lead to the resuspension of the settled solids. As they do not remove nutrients and pathogens, conventional septic tanks are commonly considered the major sources of pathogens and nutrient pollutants in the environment (Richard et al., 2016). Therefore, effluents of the conventional septic tanks are not safe for direct reuse (Beavers, 2002). To utilize such effluents, e.g. for lawn irrigation, a comprehensive post-treatment is needed. Kuwait Institute for Scientific Research (KISR) has studied a number of add-on systems that can be used in the reclamation of effluents of conventional septic tank. This paper compares the irrigation qualities of the final product water of two add-on treatment systems studied by KISR, namely, aerobic- and anaerobic-based add-on post-treatment systems. As shown in Figure 1, the aerobic-based system consists of an aerobic bioreactor, a clarifier, a saturated grass bed filter, and a chlorination unit. The anaerobic-based treatment system is presented in Figure 2. It consists of a sediment tank, a sand-gravel partially anaerobic bioreactor, a saturated grass-bed filter, and a chlorination unit. These systems were designed in a way that oxidation of the organic matter will take place in the (an)aerobic bioreactors, while the removal of the suspended solids will occur in the sedimentation tanks (clarifiers). The saturated grass-bed filter unit added for the removal of nutrients and some microorganisms (Van Buuren et al., 1998). However, all the remaining microorganisms are expected to be killed (disinfected) in a chlorination unit. Line 1 from Storage tank 2 Air Clarifier Chlorination Saturated grass filter Recycle Aerobic bioreactor Wasted sludge To storage Pumping inflow Figure 1: The aerobic-based post-treatment system (Abusam et al., 2014)

66 Abusam, Ahmed, and Mydlarczyk 2016 Comparison of Irrigation Qualities Sedimentation tank Effluent recycle to septic tank Saturated Grass Filter Chlorination Sand-gravel partially anaerobic bioreactor Pumping inflow from septic tank outlet To storage tank Figure 2: The anaerobic-based post treatment system (Abusam et al., 2014) 2. Material and Methods The two add-on reclamation systems were operated using the effluent of a full-scale conventional septic tank as an inflow. They were operated at inflows of 0.5, 0.8, 1.1, 1.4, and 1.6 l/min. and flow recycling rates of 25%, 50%, 75%, and 100%. Quality of the septic tank effluents used in the tests is presented in Table 1. At steady state conditions, weekly samples of the system s final product water were collected and analyzed according to APHA (2012). The temperature (Temp.), hydrogen concentration (ph), dissolved oxygen (DO), and electrical conductivity (EC) were measured in situ, while the following parameters were determined in the laboratory: five-days biological oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), total dissolved solids (TDS), oil and grease (O&G), ammonia nitrogen (NH 4-N), nitrate nitrogen (NO3-N), total Kajeldahl nitrogen (TKN), total nitrogen (TN), phosphate (PO 4), hydrogen sulfide (H 2S), phenol (C6H5OH), total coliform (TC), fecal coliform (FC), fluoride (F), boron (B), cadmium (Cd), lead (Pb), mercury (Hg), zinc (Zn), arsenic (As), aluminum (Al), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), and magnesium (Mg). Notice that break-point chlorination was used to disinfect the final effluents. Tables 2 and 3 present the WHO guidelines, which was used as a criteria for the comparison. 3. Results and Discussion The quality of the final product waters of systems 1 and 2 are shown in Table 4. The following sections compare and discuss the quality parameters of the two final product waters. It also briefly discusses the technological options for the reclamation of effluents of conventional septic tank systems. 3.1 ph Table 4 shows that the mean ph values of the final product water of the two add-on treatment systems were exactly the same (7.2 +/- 0.2) and that they were within the range (6.5 to 8.0) recommended by WHO. Notice that ph outside this range may adversely affect the plants through nutritional imbalance or toxicity. Irrigation water with a ph value that is lower than 6.5 usually promotes leaching of heavy metals, whereas that with ph value higher than 11 destroys bacteria and can also temporarily inhibit movement of heavy metals (WHO, 2006). 3.2 Salinity Hazards Highly saline water generally damages soil, plants, groundwater, and/or crops. Accordingly, it is generally recommended that salinity of the irrigation water should not be very high. Table 4 shows that salinity measured as EC and TDS of the final product waters of system 2 were slightly lower than that of system 1. However, according to WHO standards (Table 3), both effluents require a high degree of restriction with respect to EC. They also both require a

International Journal of Environment and Sustainability, 2016, 5(1): 64-70 67 slight to moderate degree of restriction with respect to TDS. Table 1 Quality of the Full-scale Septic Tank Effluents Used as Feed Water Parameter Add-on System 1 Add-on System 2 Mean Standard Mean Standard Temperature 21.6 1.8 26.1 4.3 ph 7.7 0.2 7.7 0.1 EC 835.9 161 789.2 49.9 TSS 14.3 5 11.1 2.2 TDS 304.9 38 267.6 13.4 NO3-N 1.1 0.7 44.7 14.8 NH4-N 56.2 15.8 1.7 2 TKN 57.9 16.3 46.1 15.2 TN 58 16.8 48.2 14.4 PO₄ 5.3 1.3 4.9 1 COD 37.8 12.4 35.8 8 BOD5 14.8 4.6 11.7 2.4 DO 1.4 0.4 1.8 0.3 O&G 0.1 0.3 0 0 TC 1.40E+07 3.00E+07 8.9E+6 2.0E+7 FC 1.20E+06 2.60E+06 1.1E+6 2.6E+6 C6H5OH 0.1 0.05 0 0 F 0 0.03 0.5 0.2 H2S 0.3 0.25 0 0 Al 0.7 0.3 0.6 0.3 As 0 0 0 0 B 0.3 0.2 0.3 0.2 Cd 0 0.05 0 0 Cu 0.1 0.2 0 0 Fe 0.3 0.5 0.2 0 Pb 0.1 0.2 0 0 Mn 0.1 0.1 0.4 0.4 Hg 0 0 0 0 Ni 0.1 0.1 1 0.8 Zn 0.1 0.1 0.7 0.5 Mg 15 6.8 13 3.5 3.3 Total Suspended Solids As given in Table 4, the anaerobic-based treated system resulted in significantly lower TSS concentrations (mean = 7.4 mg/l) in the final product water compared to that of the aerobicbased system (mean = 16.4 mg/l). According to the WHO (2006), irrigation with such waters does not require any restriction. It is generally recommended to avoid irrigation with waters that have high concentrations of suspended solids that may cause problems such as clogging of the irrigation systems, sealing of the soil surface, filling the voids between sand particles, reducing soil infiltration, and increasing soil compaction. Table 2 Guidelines for Interpretation of Water Quality for Irrigation (WHO, 2006) Potential Irrigation Problem Unit Degree of Restriction None Slight to Moderate Severe EC μs/m <0.7 0.7-3.0 >3.0 TDS mg/l <450 450-2000 >2000 TSS mg/l <50 50-100 >100 EC at SAR = 0-3 ds/m >0.7 0.7-0.2 <0.2 EC at SAR = 3-6 ds/m >1.2 1.2-0.3 <0.3 EC at SAR = 6-12 ds/m >1.9 1.9-0.5 >0.5 EC at SAR = 12-20 ds/m >2.9 2.9-1.3 <1.3 EC at SAR = 20-40 ds/m >5.0 5.0-2.9 <2.9 Sodium (Na+): meq/l <3 3-9 >9 Sprinkler Irrigation Chloride (Cl ): meq/l <3 >3 Sprinkler Irrigation Chloride (Cl ): meq/l <4 4-10 >10 Surface Irrigation Bicarbonate (HCO3) mg/l <90 90-500 >500 Boron (B) mg/l <0.7 0.7-3.0 >3.0 Total Nitrogen (TN) mg/l <5 5-30 >30 ph - Normal range: 6.5-8.0 Electrical conductivity=ec; total dissolved solids=tds; total suspended solids=tss; SAR=sodium absorption ratio 3.4 Organic Matter Organic contents of wastewater usually increase soil moisture, retain metals, and enhance microbial activity. Therefore, irrigation with wastewater is generally better than irrigating with potable water and then adding synthetic fertilizers (Ayers and Westcot, 1985). Table 4 shows that both post-treatment systems resulted in a final product that satisfies WHO s standards. It also shows that the anaerobic-based system was more efficient in removing chemical oxygen demand (COD) and biological oxygen demand (BOD 5).

68 Abusam, Ahmed, and Mydlarczyk 2016 Comparison of Irrigation Qualities Table 3 Recommended Maximum Concentrations of Trace Elements (Ayers and Westcot, 1985) Trace Element Al 5.0 Cd 0.01 Cr 0.10 Co 0.05 Cu 0.20 Fe 1.0 Pb 5.0 Mn 0.20 Ni 0.20 Zn 2.0 Table 4 Maximum Concentration (mg/l) Quality of the Final Product Waters of Systems 1 and 2 Parameter Process 1 Process 2 Mean Standard Mean Standard ph 7.6 0.2 7.6 0.2 EC (µs/m) 9.42 3.4 8.24 0.32 TSS 16.4 1.8 7.4 2.1 TDS 648.9 853.4 316 29.1 NO3-N 5.4 3.7 37.4 15.9 NH4-N 3.8 1.5 2.9 2 TKN 1.6 1.5 37.9 16.1 TN 4.8 3.5 41.3 15.3 PO₄ 8.8 4.1 4.7 0.9 COD 53.1 60.6 27.5 4.3 BOD5 24 27 9.3 2 DO 2.2 0.6 1.1 0.3 O&G 0 0 0 0 C6H5OH 0.02 0.02 0 0 F 0.4 0.24 0.4 0.2 H2S 0.01 0.012 0 0 Al 0.29 0.2 0.6 0.3 As 0 0 0 0 B 0.14 0.13 0.2 0.2 Cd 0 0 0 0 Cu 0.03 0.04 0 0 Fe 0.31 0.37 0.2 0 Pb 0.13 0.15 0 0 Mn 0.1 0.1 0.4 0.4 Hg 0 0 0 0 Ni 0.24 0.52 1.1 0.9 Zn 0.08 0.07 0.6 0.4 Mg 16.38 8.1 19.4 3.7 3.5 Total Nitrogen Nitrogen is an essential macronutrient for plant growth. Nitrogen is usually found in wastewater as ammonia, nitrite, nitrate and/or organic nitrogen compounds. Total nitrogen (TN) is the sum of all these different forms of nitrogen. According to the WHO guidelines (Table 3), no degree of restriction is required when TN is less than 5 mg/l while a slight to moderate degree of restriction is required for irrigation when TN between is between 5 to 30 mg/l. Generally, TN less than 30 mg/l does not harm plants, except sensitive crops such as sugar beets (Ayers and Westcot, 1985). As given in Table 4, the aerobicbased post treatment system had reduced significantly TN concentrations (4.8 ± 3.5 mg/l), while that of the anaerobic post treatment system remained high (41.3 ± 15.3 mg/l). The high removal of TN of the aerobic-based system can be attributed to high nitrification (oxidation of ammonia) in the aerobic bioreactor and subsequent denitrification in the grass soil filter. It should be noticed that such high removal of nitrogen is usually not required when the reclaimed water will be used as irrigation water. 3.6 Phosphorus Phosphorus is also an important macronutrient for plants. It is commonly present in significant amounts in wastewater. Table 4 shows that the anaerobic-based system resulted in a higher removal of phosphorus than the aerobic system. Concentrations of phosphorus in the final product waters were 4.7 ± 0.9 mg/l for the anaerobic-based system and 8.8 ± 4.1 mg/l for the aerobic-based system. These results indicate that huge amounts of phosphorus, that would have been beneficially recycled through agricultural irrigation reuse, was removed by the anaerobic-based system. 3.7 Boron Boron is also an essential micronutrient needed for plant growth. However, high concentrations of boron can affect particularly ornamental plants (WHO, 2006). As presented in Table 4, there is no significant difference between boron concentration of the final product water of the two systems. Furthermore, the two final

International Journal of Environment and Sustainability, 2016, 5(1): 64-70 69 product waters clearly meet WHO guidelines for non-restricted irrigation (Table 3). 3.8 Heavy metals There are always health risks to crop consumers from irrigating using waters that are highly concentrated with heavy metals (WHO, 2006). As Table 4 shows, both of the add-on treatment systems had efficiently removed Al, B, Cu, and Zn. However, they were very poor in the removal of some heavy metals such as Fe, Pb, Mn, and Ni. In fact, concentrations of heavy metals slightly increased after post-treatment due to leaching from grass filters. Except these elements, two post-treatment systems resulted in more or less the same heavy metal concentrations that satisfy the WHO standards. 3.9 Microbiological Quality Break-point chlorination was used to disinfect the final product waters. Chlorination often results in up to six or more reduction units (99.9999%) of pathogen concentration (WHO, 2006). As expected, it resulted in the killing of almost all the pathogenic microorganisms (results are not shown here). Therefore, with respect to microbiological quality, the two product waters satisfied the WHO standards. 3.10 Reclamation Technology The above-mentioned results clearly indicate that post-treatment of conventional septic tank effluents to a degree that satisfies irrigation water quality, as specified by the WHO, requires the use of both aerobic and anaerobic biological treatments. This can be achieved through using a hybrid system in which aerobic and anaerobic are separated in space or time. The add-on system 2 used in this study is a good example of the systems that separate the aerobic-anaerobic treatment conditions in space. The sequencing batch reactor (SBR) is another example of the systems that separate the aerobic and anaerobic treatment conditions over time. Each of these technological options has it merits and demerits that should be taken into consideration. A hybrid aerobic-anaerobic system generally requires larger space. For SBR, however, availability of power (electricity) and maintenance skills are pre-requisites. Thus, the selection of post-treatment technology depends on the particular situation. 4. Conclusions Performance of an aerobic-based system as a post-treatment system for conventional septic tank effluents was compared to that of an anaerobic-based post-treatment system. Obtained results indicated that reclamation of conventional septic tank effluents requires both aerobic-anaerobic treatments since none of them can efficiently remove all pollutants of concern. Depending on the particular situation, a hybrid aerobic-anaerobic biological treatment system or a sequential batch reactor (SBR) can be the best option for the reclamation of effluents of conventional septic tanks for reuse in irrigation purposes. Acknowledgments Data used in this study were collected during the execution of a project entitled Treatment, Reclamation and Reuse of Effluent of Septic Tank Treating Small Flow of Domestic Wastewater at Site (WT027C) at KISR. This project was partially financed by the Kuwait Foundation for the Advancement of Sciences (KFAS) under the code 2012-1505-02. References APHA (2012), Standard Methods for Examination of Water and Wastewater, American Public Health Association, Washington, D.C., USA Ayers, R.S. and Westcot, D.W. (1985), Water quality for agriculture, FAO Irrigation and Drainage, Paper 29, Food and Agriculture Organization, Rome Beavers, P. (2002), On-site Sewerage Facilities Guidelines for the Use and Disposal of Grey Water in Un-Sewer Areas, Queensland Gov-

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