Performance, water quality and enviro-economic investigations on solar distillation treatment of reverse osmosis reject and sewage water

Solar Energy 173 (2018) 160–172

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Performance, water quality and enviro-economic investigations on solar distillation treatment of reverse osmosis reject and sewage water

T

⁎

K.S. Reddya, , H. Sharona, D. Krithikab, Ligy Philipb a b

Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India Environmental and Water Resources Engineering Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar distillation Wastewater treatment RO reject Enviro-economic analyses

In this work, effective treatment of RO reject and domestic sewage water in a single step using indigenously developed tilted solar distillation unit has been proposed. Behavior of the unit along with its characteristics, treated water quality, environmental benefits and economics has been reported based on experimental observations. Around, 4.79 and 4.48 L/d of treated water are produced by the unit at a thermal efficiency of 48.5% and 45.3% during RO reject and sewage water distillation, respectively. Suspended particles of re-circulated sewage water caused clogging of wick and affected tilted solar distillation unit’s performance and efficiency. Smooth operation of the unit is noticed during RO reject distillation. The proposed unit could prevent at least 23.73 tons of CO2, 158.54 kg of SO2 and 64.75 kg of NO emissions during its 20 Yr life span. Wick replacement frequency and interest rate have a signification impact on distillation unit’s treated water production cost. The proposed distillation unit’s treated water production cost is lower than basin solar stills reported in literatures. Treated water is clear, odor free and bacterial free. Physical properties and heavy metal concentrations of treated water are well within the standards for safe drinking water except BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) such that the treated water can be used for other domestic and irrigation purposes. The results obtained from this study confirm solar distillation as an effective and sustainable option for wastewater treatment.

1. Introduction Fresh water finds various livelihood applications in domestic, industrial and energy sectors of nations around the globe. The quantity and quality of these limited precious fresh water resources are degrading continuously because of over exploitation and wastewater dumping (Manju and Sagar, 2017; Rijsberman, 2006; Rajasulochana and Preethy, 2016). Wastewaters that are generally dumped into water bodies without any pre-treatment are mainly RO (reverse osmosis) reject and domestic sewage water. RO reject is a mixture of pre-treatment chemicals and concentrated feed water. Domestic sewage water represents used water from households. Around, 80.0% of water supplied to households for domestic use returns back as domestic sewage water (Kaur et al., 2012). Techniques like deep well injection and discharge into surface waters are widely followed for disposing RO reject but these methods have posed severe threats to environment (Ahmed et al., 2000). Wastewater treatment and reuse is the only available option that can close water cycle, reduce water stress and mitigate negative impacts on

⁎

environment (Vergine et al., 2017). Biological stabilization ponds (Rusan et al., 2007) and activated sludge-extended aeration plant (AlLahham et al., 2007) are widely used for treating wastewaters generated from houses and industries. However, these techniques cannot remove toxic heavy metals, nitrogen, phosphorous, organic and inorganic substances from wastewater in a single step (Rajasulochana and Preethy, 2016). Moreover, they cannot tolerate high salinity and heavy metal concentration of RO reject. Hence, evaporation technique is widely recommended for treating RO reject (Giwa et al., 2017). Energy consumption and CO2 emission per m3 of wastewater treated in Indian sewage treatment plants are in the range of 0.40–4.87 kWh and 0.78–3.04 kgCO2eq, respectively (Singh et al., 2016). Similarly, 27.4 kg of oil is required to produce 1.0 m3 of distillate (treated water) by evaporation process (Kalogirou, 2005). Pollution and treatment cost of existing biological treatment plants and fossil fuel based distillation units can be reduced by utilizing renewable wind and solar energy for their operation (Haralambopoulos et al., 1997; Han et al., 2013; Halaby et al., 2017). Lack of finance, proper infrastructures and skilled work force in wastewater treatment sectors has lead to poor wastewater

Corresponding author. E-mail address: [email protected] (K.S. Reddy).

https://doi.org/10.1016/j.solener.2018.07.033 Received 17 April 2018; Received in revised form 9 July 2018; Accepted 11 July 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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Nomenclature

Ats hfg Its ∙ md MY Qca Qcw Qew Qla Qra Qrw

Ta Tgc Tv Tw ηth ηith

aperture area of tilted solar distillation unit (m2) latent heat of evaporation (J/kg) global horizontal solar radiation intensity (W/m2) treated water production (kg/s) annual average treated water production (L) convective heat loss to ambient (W) convection heat transfer from wetted wick to glass cover (W) evaporation heat transfer from wetted wick to glass cover (W) conduction heat loss to ambient through insulation (W) radiation heat loss to ambient (W) radiation heat transfer from wetted wick to glass cover (W)

ambient temperature (°C) outer glass cover temperature (°C) air-vapor mixture temperature (°C) wetted wick temperature (°C) overall thermal efficiency (%) instantaneous thermal efficiency factor

Abbreviations

AOM factor annual operation and maintenance factor CC capital cost (USD) IR interest rate SV factor salvage value factor TAC total annualized cost (USD) TPC treated water production cost (USD/L) UMC useful material cost (USD)

water production rate and thermal efficiency. b. Water quality analysis of raw and treated water to estimate pollutant removal efficiency of solar distillation unit and to confirm the suitability of treated water for reuse and safe disposal. c. Enviro-economic analyses of tilted solar distillation unit to assess its harmful gas emission mitigation potential and treatment cost at various interest rate and operating conditions.

treatment in Asian continent (Sato et al., 2013). For example, in India, only 10.0% of sewage water is treated effectively and remaining 90.0% is either dumped into water bodies or sold to farmers for carrying out agricultural activities (Kaur et al., 2012). Long term irrigation with wastewater has caused chemo-desertification of fertile land and increased health risks to living beings (Rebhun, 2004; Pereira et al., 2002; Singh et al., 2012). Water starved regions of the globe have abundant solar radiation potential and it can be tapped for wastewater treatment (Shatat et al., 2013) and water pasteurization (Duff and Hodgson, 2005). Basin solar stills are capable of producing parasite and Arsenic free drinking water from polluted water (Onyegegbu, 1984; Jasrotia et al., 2013). Dewatering of wastewater sludge (Haralambopoulos et al., 2002) and recovery of antioxidants from oil mill wastewater (Sklavos et al., 2015) have also been successfully carried out using basin solar still. The distillate obtained from basin solar still has 80.0% lower COD (Chemical Oxygen Demand) and 90.0% lower TKN (Total Kjeldhal Nitrogen) compared to raw oil mill wastewater (Potoglou et al., 2003). (Velmurugan et al., 2008, 2009; Farahbod et al., 2013) have treated industrial effluent using basin solar stills. Asadi et al. (2013) used stepped solar still for distilling kitchen and palm oil wastewaters. However, contamination of condensate with polluted water or wastewater is highly possible in basin solar stills during feed water addition to basin (Hanson et al., 2004). From above literatures, it could be inferred that solar distillation is an effective technique for wastewater treatment and basin solar stills have been widely used for this purpose. However, studies dealing with solar distillation of raw domestic wastewaters (sewage water and RO reject) which are the major source for fresh water body pollution are very scarce. Hence, in the present research, feasibility of solar distillation technique for sewage water and RO reject treatment using indigenously developed tilted solar distillation unit with reject recirculation has been explored experimentally. Low area occupancy (Tiwari and Somwanshi, 2018) and reduced chance of condensate contamination with feed water (as flow is only by capillary action) makes tilted solar distillation unit more competitive and superior to basin solar stills. The major objectives of the present research work are as follows:

2. Tilted solar distillation unit – System description and operating principle The tilted solar distillation unit developed for sewage water and RO reject distillation is presented schematically in Fig. 1. It consists of insulated stainless steel distillation chamber, aluminium wastewater trough, tempered glass cover, treated water collection trough and necessary provisions for treated water and reject water drain (Sharon et al., 2017). Absorber surface of distillation chamber is lined with black blended woolen wick of porosity 66.0%. Blended woolen wick used in this study is nothing but felt sheet which is very slow to deterioration and wear (Natindco, 2017). Moreover, it can be polished and reused (Natindco, 2017) which makes it highly suitable for wick based solar wastewater distillation process. The distillation unit is placed at an tilt angle of 13° (latitude of Chennai) from horizontal over an mild steel frame facing due south direction to trap maximum solar radiation throughout the year. Tilt angle closer to the latitude of corresponding site is considered to be optimum for higher year round distillate production in solar stills due to minimal reflection of incident solar radiation from glass cover (Khalifa, 2011). Overhead tank of 40.0 L capacity is used to store and supply wastewater to aluminium trough kept inside the distillation chamber. One end of black blended woolen wick is immersed inside aluminum wastewater trough such that the wastewater to be treated gets distributed uniformly over the wick by capillary action. The aperture area of distillation unit is around 1.18 m2. Specifications of important parts associated with tilted solar distillation unit are presented in Table S1 of supplementary material. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.solener.2018.07.033. Transparent tempered glass cover of solar distillation unit facilitates passage of solar radiation through it during sunshine hours, as a result wetted wick lined absorber surface gets heated up and water vapors are formed. Condensation of formed vapors occurs over the inner surface of

a. Performance assessment of tilted solar distillation unit during raw sewage water and RO reject distillation by estimating its treated

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Fig. 1. Tilted solar distillation unit - schematic representation.

3. Wastewater collection

tempered glass cover which is relatively at a lower temperature than absorber surface (Mathioulakis and Belessiotis, 2003). These condensed vapors move down towards the lower edge of glass cover by gravity and are collected in treated water collection trough. Excess unevaporated wastewater leaves the unit as reject through the drain kept at the lower end of distillation chamber. Schematic of tilted solar distillation unit indicating associated heat transfer processes (Abujazar et al., 2018), fluid flow directions and thermocouple locations are depicted in Fig. 2.

RO reject is collected from Reverse Osmosis (RO) plant located in Brahmaputra hostel zone of Indian Institute of Technology Madras (IIT Madras), Chennai, India. The capacity of RO plant is around 3000.0 kg/ h and it purifies well water and metro water with a recovery ratio of 50.0%. Sewage water used for experimentation is collected from mixing chamber of biological sewage treatment plant of 5.0 ML/d capacity

Fig. 2. Schematic of tilted solar distillation unit indicating necessary energy transport processes and temperature measuring locations.

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defined as the ratio of energy utilized for treated water production to cumulative solar radiation incident over the aperture of solar distillation unit and is given by Kabeel et al. (2017) and Manokar et al. (2017),

located inside IIT Madras. In mixing chamber, kitchen and toilet wastewaters from hostels, labs and staff apartments are mixed together and finally supplied to biological sewage treatment unit for treatment. On contrary to RO reject, sewage water contains suspended particles. Nearly, 60.0 L of RO reject and 60.0 L of sewage water are collected separately in high density polyethylene cans a day before the start of respective experiments.

∙

ηth =

∑ (md × hfg ) ∑ (Its × Ats )

× 100

(1)

Maximum error (%) is determined by Srithar et al. (2016),

Accuracy of the instrument × 100 Minimum value of the output measured

4. Experimentation on tilted solar distillation unit

Max. error(%) =

Experiments are conducted on tilted solar distillation unit kept over the terrace of Mechanical Engineering Department, IIT Madras, Chennai (13.08°N, 80.27°E) with RO reject from 22nd to 25th February 2017 and with sewage water from 2nd to 5th March 2017 by incorporating recirculation. Recirculation of unevaported reject and refilling of wastewater trough with wastewater from overhead tank are carried out manually for every 30 mins until sunset. Global horizontal solar intensity, temperature of different components of tilted solar distillation unit and hourly treated water production are measured every fifteen minutes during each experimental day from 7:00 h to 18:00 h. Ambient temperature and wind velocity data are obtained from nearby weather monitorng station. K-type thermocouples calibrated in Central Electronics Centre (CEC), IIT Madras are used for measuring outer glass cover (3 nos), air-vapor mixture (3 nos) and black blended woolen wetted wick (3 nos) temperatures. The obtained temperature readings are processed and stored in personal computer using data logger connected to the computer. Solar light pyranometer provided with inbuilt data logger is used to measure and record global horizontal solar intensity. Treated water obtained by solar distillation is collected in plastic beaker and is subjected to water quality analysis. Photograph of tilted solar distillation unit used for experimentation is shown in Fig. 3. Treated water production and thermal efficiency are used to assess the performance of tilted solar distillation unit. Thermal efficiency is

List of instruments and their maximum error (%) during experiments is tabulated in Table S2 of supplementary material. Standard methods as suggested by American Public Health Association (APHA, 2012) are followed for wastewater and treated water quality analyses. Description of the methods used for water quality analysis can be found in Section S1 of supplementary material.

(2)

5. Performance and treated water quality analyses of tilted solar distillation unit In this section, performance of tilted solar distillation unit during RO reject and sewage water distillation along with water quality analyses results have been discussed. Peak ambient temperature of 37.0 °C and 39.0 °C is recorded during experiments with RO reject and sewage water, respectively. Peak wind velocity of 5.1 m/s is noticed at 15:00 h and 17:00 h on 23rd February 2017 and 3rd March 2017, respectively. The hourly ambient conditions noticed during experiments with RO reject and sewage water is tabulated in Tables S3 and S4 of supplementary material. 5.1. Performance of tilted solar distillation unit Behaviour of tilted solar distillation unit distilling wastewaters along with its characteristics has been discussed in this section. RO

Fig. 3. Photograph of tilted solar distillation unit used for wastewater distillation. 163

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Fig. 4. Hourly temperature, hourly solar radiation & hourly treated water production profile of solar distillation unit during RO reject distillation.

evaporation rate during these hours. Due to enhanced evaporation during peak hours, accumulation of latent heat of evaporation and radiation heat losses from evaporating to condensing surface occurs as a result air-vapor mixture temperature higher than wetted wick temperature by 2.0–4.7 °C is observed during RO reject distillation (Sakthivel et al., 2010). High air-vapor mixture temperature during peak hours has also been observed by Khalifa and Hamood (2009) in basin solar still and by El-Agouz et al. (2015) in inclined solar still. During sewage water distillation, air-vapor mixture temperature is nearly closer to or lower than wetted wick temperature. Moreover, the wetted wick temperature during sewage water distillation is higher than water sterilization temperature (65.0 °C) for about four to five hours which favours efficient killing of bacterias and microbes even in un-evaporated sewage water (Hameed and Ahmad, 1997; Saitoh and ElGhetany, 1999). Wetted wick temperature above 100.0 °C is noticed from 13:30 h to 15:30 h on 4th March 2017. This observation can be attributed to combined effect of high solar radiation and poor wetting of wick due to deposition of suspended particles in recirculated sewage water. During experiments, hourly treated water production profile follows solar radiation intensity profile for the corresponding days. Treated water production starts immediately after incidence of solar radiation over wetted wick. This is mainly due to its lower heat capacity which favors quick warm up and immediate start of evaporation (Xiao et al., 2013). During late evening hours, hourly treated water production falls in line with solar radiation intensity, which is contrary to the behavior of conventional deep basin solar stills where nocturnal productivity is

reject is clear and saline while sewage water is more turbid and contains tiny suspended particles. Cumulative solar intensity during distillation of RO reject and sewage water is in the range of 19.08–20.70 MJ/m2-d and 16.29–21.33 MJ/m2-d, respectively. 5.1.1. Solar distillation of RO reject and sewage water - hourly temperature profile and treated water production Hourly variation of temperature and treated water production rate along with global solar intensity during solar distillation of RO reject and sewage water is presented in Figs. 4 and 5, respectively. The hourly temperature and treated water production rate increases with increase in solar radiation due to enhanced heating effect of solar radiation absorbed by wetted wick. Water vapors released from wetted wick condenses over the inner surface of glass cover by releasing its latent heat, as a result glass cover temperature increases (Sahoo et al., 2008). However, glass cover temperature is lower than wetted wick and airvapor temperatures throughout the experiments because of its transparency and low thermal inertia (Sadineni et al., 2008; Onyegegbu, 1986). Air-vapor mixture temperature observed during RO reject distillation is little bit lower than wetted wick temperature during morning and evening hours. This observation is contrary to air-vapor mixture temperature profile noticed during ordinary tap water distillation (Sharon et al., 2017) which can be attributed to salinity of RO reject which increases boiling point elevation and reduces air-vapor mixture temperature. Highest glass cover, air-vapor mixture and wetted wick temperatures are observed during noon because of increased solar radiation and

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Fig. 5. Hourly temperature, hourly solar radiation & hourly treated water production profile of solar distillation unit during sewage water distillation.

presented in Fig. 6a and b, respectively. Treated water production and thermal efficiency of the unit increases with increase in cumulative solar radiation during RO reject distillation and are in the range of 4.12–4.79 L/d and 43.4–48.5%, respectively. Even though, un-evaporated RO reject is re-circulated there is no significant drop in performance of the distillation unit which may be due to low salinity and low turbidity of RO reject (4452.43 mg/L and 1.34 NTU). Highest and lowest diurnal treated water production during sewage water distillation is around 4.48 and 3.57 L/d on 3rd and 5th March 2017, respectively. Nocturnal treated water production during experiments is only around 4.0% of total treated water production, as most of the heated feed water leaves the distillation unit throughout the day leading to reduced availability of heated water mass inside the distillation unit for further evaporation after sunset hours. During high solar intensity, enhanced evaporation of sewage water takes place, as a result tiny suspended particles of re-circulated sewage water gets dried up and settles over the wick which lowers the capillary action and further wetting of wick. Hence, further evaporation process slows down and leads to reduced overall treated water production and performance of the unit. Higher thermal efficiency of 46.8% is noticed on 2nd March 2017 which has lowest cumulative solar intensity of 16.29 MJ/m2-d while lowest thermal efficiency of 40.5% is noticed on 4th March 2017 which has highest cumulative solar intensity of 21.33 MJ/m2-d. This observation is contrary to results obtained for RO reject distillation which justifies the clogging effect of wick during sewage water distillation. In order to prevent formation of dry patches due to deposition of

higher (Onyegegbu, 1986). This observation of tilted solar distillation unit is due to continuous removal of un-evaporated heated wastewater (RO reject, sewage water) from the unit as reject which causes unavailability of heated feed water for further evaporation during nonsunshine hours. Similar kind of profile has also been observed by Sodha et al. (1981) and Hansen et al. (2015) during their experiments with wick type solar stills. The overall summary of experimental results and climatic conditions during solar distillation of RO reject and sewage water is tabulated in Table 1. Experimental days are mostly clear and sunny during RO reject distillation. Cloud passage is observed during experiments with sewage water. Daily average air-vapor mixture temperature is lower than daily average wetted wick temperature. Highest daily average hourly treated water production rate during RO reject and sewage water solar distillation is 0.42 L/h and 0.39 L/h, respectively. For a cumulative solar radiation intensity of ∼19.90 MJ/m2-d, higher wetted wick temperature and lower daily average treated water production are noticed during sewage water distillation (3rd March 2018) in comparison to RO reject distillation (25th February 2018). This observation confirms poor wetting of wick due to clogging caused by suspended particles of sewage water which leads to enhanced absorber surface temperature and reduced overall evaporation rate. 5.1.2. RO reject and sewage water distillation - diurnal treated water production and thermal efficiency Diurnal treated water production and thermal efficiency of tilted solar distillation unit during RO reject and sewage water distillation is

165

166 Clear Nil

Rainfall

0.76

0.72

Nil

Clear

Sunny

0.38

46.4 ± 2.3

55.8 ± 0.2

57.7 ± 3.6

3.0

29.6

0.37

Clouds

Daily average Highest

Daily hourly treated water productivity (L/h)

46.5 ± 2.4

Sunny

Daily average

Glass cover temperature (°C)

55.8 ± 0.2

Climate

Daily average

Air-vapor mixture temperature (°C)

57.2 ± 3.5

2.6

Daily average

Average wind velocity (m/s)

Wetted wick temperature (°C)

29.7

Average ambient temperature (°C)

867.0 19.20

864.0 19.08

Cumulative solar radiation (MJ/m2-d)

474.0

471.0

Solar radiation intensity (W/m2)

Daily average Highest

22nd February 2017

Date of experiments 23rd February 2017

RO reject

Type of wastewater

Nil

Clear

Sunny

0.82

0.41

46.0 ± 2.3

55.9 ± 0.4

58.8 ± 3.7

2.3

32.1

20.70

924.0

511.0

24th February 2017

Nil

Nearly Clear

Sunny

0.86

0.42

44.1 ± 2.0

53.9 ± 0.5

57.6 ± 3.7

2.4

31.8

19.86

918.0

490.0

25th February 2017

Table.1 Summary of experimental results and climatic condition during solar distillation unit of RO reject and sewage water.

Nil

Passage during noon

Sunny

0.72

0.33

40.8 ± 1.9

50.3 ± 0.5

53.8 ± 3.7

2.7

30.9

16.29

821.0

401.0

2nd March 2017

Sewage water

Nil

Nearly clear sky

Sunny

0.75

0.39

45.2 ± 2.1

56.4 ± 0.8

61.0 ± 2.6

2.6

32.2

19.90

872.0

491.0

3rd March 2017

Nil

Intermittent passage during morning hours

Sunny

0.85

0.38

47.3 ± 2.5

61.9 ± 0.8

66.8 ± 4.4

2.9

33.4

21.33

937.0

526.0

4th March 2017

Nil

Frequent passage during morning hours

Sunny

0.67

0.32

46.9 ± 1.8

59.2 ± 1.2

60.2 ± 1.7

3.6

32.4

17.09

898.0

421.0

5th March 2017

K.S. Reddy et al.

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Fig. 6. Diurnal treated water production and thermal efficiency of tilted solar distillation unit during (a) RO reject and (b) Sewage water distillation.

suspended particles over the evaporating surface, feed water flow rate must be at least 1.3 times higher than evaporation rate (Chong Tze-Ling et al., 2014). In the present investigations, wetting of wick is achieved only by capillary effect hence required feed water flow rate to prevent formation of dry patches is not achieved. In addition, liberation of trapped non-condensable gases like CO2, N2, CH4, NH3 and H2S (Sullivan and Krieger, 2001) occurs during sewage water evaporation which also reduces distillation unit’s performance by increasing air resistance. Hence, operation at lower temperature and evacuation at regular intervals (Al-Hussaini and Smith, 1995) must be followed for enhanced performance of solar distillation unit treating sewage water.

thermal efficiency of the unit distilling RO reject is found to increase steeply with increase in (Tv − Ta)/It. Instantaneous thermal efficiency of the unit distilling sewage water is higher than the unit distilling RO reject for lower values of (Tv − Ta)/It (up to 0.045). For values of (Tv − Ta)/It above 0.045, instantaneous thermal efficiency decreases which may be due to clogging of wick and evolution of non-condensable gases at high solar radiation. Reduced instantaneous thermal efficiency of unit distilling RO reject at lower values of (Tv − Ta)/It in comparison to the unit distilling sewage water is due to high salinity of RO reject (4452.43 mg/L) in comparison to sewage water (1371.30 mg/L). Positive value of y-intercept for both the curves confirms start of evaporation process immediately after solar radiation incidence over the evaporating surface (Dev and Tiwari, 2009).

5.1.3. Characterization of tilted solar distillation unit The characteristic curve of tilted solar distillation unit plotted using average values obtained during peak sunshine hours (ie.,) 10:00–14:00 h of experimentation periods is presented in Fig. 7. The positive slope of characteristic curve confirms higher evaporative heat loss from distillation unit with increase in temperature difference and solar intensity (Kumar et al., 2014; Galvez et al., 2009). Instantaneous

5.2. Water quality analysis – Wastewater and treated water The quality results of treated water obtained by solar distillation of RO reject and Sewage water along with necessary standards and 167

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Fig. 7. Characteristic curve of tilted solar distillation unit distilling RO reject and sewage water.

COD other qualities of treated water obtained by solar distillation of RO reject are well within drinking water standards. Water quality of treated water obtained by solar distillation of sewage water is very much better than standards proposed for safe disposal of effluent into inland surface waters (CPCB, 1986). BOD, COD, turbidity and bacteria removal efficiency are around 97.3%, 95.8%, 98.3%, and 100.0% for tilted solar distillation unit distilling sewage water. COD removal efficiency of tilted solar distillation unit is higher than the removal efficiency of basin solar still studied by Potoglou et al. (2003). Treated water is very much clear which could be seen from Fig. S1 in supplementary material and can be justified by its turbidity value reported in Table 3.

removal efficiency is tabulated in Tables 2 and 3 respectively. The pH value of treated water obtained by solar distillation of RO reject is within drinking standards. Salinity, chloride content and electrical conductivity of the treated water are very much lower than wastewaters which confirm effective removal of ions by solar distillation process (Flendrig et al., 2009). According to WHO (World Health Organization) guidelines, BOD and COD values for safe drinking water are 2.0 and 10.0 mg/L (WHO, 2004). BOD and COD content of treated water obtained by solar distillation of RO reject are little bit higher than the drinking water standards, which stress the need for some post treatment processes to make the distillate more suitable for drinking. Treated water is free from total coliform and fecal coliform. Except, BOD and

Table 2 Quality results of RO reject, treated water along with removal efficiency of tilted solar distillation unit. S. No.

Parameter

RO reject

Treated water (22nd Feb 2017)

Treated water (25th Feb 2017)

Indian standards (BIS, 2012)

Removal efficiency (%)

1 2 3 4 5

pH EC (µS/cm) Salinity (mg/L) Turbidity (NTU) Total hardness (as CaCO3) (mg/L) Chlorides (as Cl) (mg/L) Total solids (mg/L) Total suspended solids (mg/L) Total dissolved solids (mg/L) Calcium (as Ca+) (mg/L) Sulphate (as SO42−) (mg/L) Nitrate (as NO3–) (mg/L) Alkanity (as CaCO3) (mg/L) Magnesium (as Mg) Phosphate (as PO43−) (mg/L) Total nitrogen (mg/L) BOD (mg/L) COD (mg/L) Bacterial quality Total coliform (MPN/100 mL) Fecal coliform (MPN/100 mL)

6.91 ± 0.05 1127.00 ± 2.00 4452.43 ± 5.86 1.34 ± 0.06 806.70 ± 11.5

7.21 ± 0.21 53.04 ± 0.19 19.29 ± 0.04 0.92 ± 0.02 13.40 ± 5.77

7.32 ± 0.17 61.40 ± 0.32 20.61 ± 0.10 0.99 ± 0.03 20.00 ± 5.10

6.50–8.50 NA NA 5.0 300.00

– 95.3 99.6 31.3 98.3

2466.70 ± 5.86 1106.70 ± 20.2 104.40 ± 10.5 1002.30 ± 0.58 23.54 ± 0.78 823.40 ± 25.2 56.60 ± 4.14 420.00 ± 52.9 26.48 ± 0.25 5.14 ± 0.42 69.83 ± 0.81 95.60 ± 4.87 353.40 ± 30.5

10.67 ± 0.04 52.50 ± 5.46 6.65 ± 5.34 46.88 ± 0.11 1.66 ± 0.82 14.80 ± 1.28 3.38 ± 0.53 43.40 ± 5.77 1.25 ± 0.88 0.14 ± 0.06 10.22 ± 0.28 15.30 ± 3.05 38.90 ± 1.10

11.40 ± 0.10 59.80 ± 3.00 6.12 ± 2.38 54.62 ± 0.115 4.33 ± 0.35 14.90 ± 0.46 3.24 ± 0.33 53.40 ± 5.77 3.60 ± 0.99 0.11 ± 0.01 9.94 ± 0.06 19.30 ± 1.15 32.70 ± 6.20

250.00 NA NA 500.00 75.00 200.00 45.00 200.00 30.00 NA NA NA NA

99.6 95.3 93.6 95.3 92.9 98.2 94.0 89.7 95.3 97.3 85.4 83.9 88.9

12.00 < 1.8

< 1.8 < 1.8

< 1.8 < 1.8

Nil Nil

100.0 100.0

0.05* 5.00* 0.05* 0.05*

66.7 32.9 82.1 60.9

6 7 8 9 10 11 12 13 14 15 16 17 18 19

Heavy metals 1 Lead (Pb) (mg/L) 2 Zinc (Zn) (mg/L) 3 Manganese (Mn) (mg/L) 4 Copper (Cu) (mg/L)

0.069 0.140 0.078 0.023

± ± ± ±

0.002 0.056 0.007 0.005

0.023 0.098 0.014 0.009

± ± ± ±

0.005 0.023 0.007 0.002

0.030 0.094 0.020 0.010

< 1.8 – less than detectable limit; NA – not available. * Drinking Water Standards (Purdom, 1980). 168

± ± ± ±

0.008 0.025 0.005 0.002

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Table 3 Quality results of sewage water, treated water along with removal efficiency of tilted solar distillation unit. S.No

Parameter

Sewage water

Treated water (2nd Mar 2017)

Treated water (5th Mar 2017)

Safe disposal standards (CPCB, 1986)

Removal efficiency (%)

1 2 3 4 5 6 7

pH EC(µS/cm) Salinity (mg/L) Turbidity (NTU) Chlorides (as Cl) (mg/L) Total solids (mg/L) Total suspended solids (mg/L) Total dissolved solids (mg/ L) Sulphate (as SO42−) (mg/ L) Nitrate (as NO3–) (mg/L) Potassium (as K+) (mg/L) Phosphate (as PO43−) (mg/ L) Total nitrogen (mg/L) BOD (mg/L) COD (mg/L) Bacterial quality Total coliform (MPN/ 100Ml) Fecal coliform (MPN/ 100Ml)

7.23 ± 0.02 1492.30 ± 2.08 1371.30 ± 23.11 55.10 ± 2.89 759.71 ± 23.11 1916.70 ± 25.2 588.40 ± 21.9

7.86 ± 0.04 173.50 ± 0.26 150.68 ± 0.45 0.91 ± 0.09 83.46 ± 0.45 183.20 ± 4.97 28.80 ± 4.48

7.34 ± 0.02 189.50 ± 0.58 162.50 ± 1.84 0.59 ± 0.04 90.01 ± 1.84 186.20 ± 4.95 17.60 ± 4.75

5.50 – 9.00 NA NA 5.0** 250.00** NA 100.00

– 88.4 89.0 98.3 89.0 90.4 95.1

1328.40 ± 3.24

154.40 ± 0.49

168.60 ± 0.20

500.00**

88.4

142.50 ± 5.37

19.26 ± 1.26

22.70 ± 3.33

200.00**

86.5

18.56 ± 2.66 27.49 ± 0.45 28.23 ± 2.31

2.11 ± 0.03 2.80 ± 0.61 0.06 ± 0.02

2.13 ± 0.02 1.31 ± 0.88 0.14 ± 0.05

10.00 NA 5.00

88.6 89.8 99.8

60.80 ± 0.29 506.70 ± 23.1 913.40 ± 30.6

22.50 ± 1.12 12.50 ± 0.42 38.70 ± 1.12

39.12 ± 0.47 10.00 ± 0.20 43.80 ± 1.25

50.00 30.00 250.00

62.9 97.3 95.8

40000000.00

2.00

2.00

≤100.00

99.9

22000.00

Nil

Nil

≤100.00

100.0

0.10 5.00 2.00 3.00

70.6 51.1 68.9 57.1

8 9 10 11 12 13 14 15 16

Heavy 1 2 3 4

metals Lead (Pb) (mg/L) Zinc (Zn) (mg/L) Manganese (Mn) (mg/L) Copper (Cu) (mg/L)

0.102 0.092 0.029 0.070

± ± ± ±

0.023 0.054 0.008 0.047

0.030 0.045 0.009 0.030

± ± ± ±

0.002 0.010 0.007 0.002

0.042 0.045 0.012 0.042

± ± ± ±

0.004 0.023 0.008 0.012

NA – not available; ** Drinking water standards as per (BIS, 2012) ;

developed alternate systems.

Both treated water and preheated un-evaporated sewage water are odor free throughout the experiments. Similar observation has been recorded by Qasim (1978) while distilling sewage water using basin solar still. (Wang et al., 2007) noticed odor in biologically treated and disinfected wastewater which justifies the necessity and superiority of solar distillation process for treating sewage water. Except BOD and COD, all other quality values of treated water obtained by solar distillation of sewage water are within drinking water standards hence the treated water can be used for domestic purposes like bathing, washing and flushing (Hingorani, 2011). No health related issues have been noticed for persons handling biologically treated wastewater during a fourteen-month study conducted by Wang et al. (2007). Hence, superior treated water obtained by solar distillation of sewage water is very safe and can be used for irrigation without any health risks. Heavy metal compositions of RO reject used for experimentations are within the standards for safe disposal. Lead concentration of sewage water is higher than safe disposal standard. Heavy metals concentration of treated water obtained in a single step by solar distillation of RO reject and sewage water are far better than the proposed safe drinking water and safe disposal standards.

6.1. Environmental benefit analysis Stainless steel, tempered glass, mild steel, brass, aluminium and glass wool based items are used for fabrication of tilted solar distillation unit (Sharon et al., 2017). During fabrication and development of tilted solar distillation unit, energy from coal based power plants is utilized. However, during its operation only renewable solar thermal energy is used as a result energy spent on the system can be regained. The time span taken to regain the spent energy is termed as energy payback time and is given by Kumar and Kurmaji (2013),

Energy payback time (Yr ) energy density of ⎤ ⎞ mass of each component ⎤ × ⎡ ∑ ⎛⎜ ⎡ ⎟ ⎢ ⎢ ⎣ of solar distillation unit ⎥ ⎦ ⎣ each component ⎥ ⎦⎠ ⎝ = ⎡ Annual energy output from distillation unit ⎤ ⎢ ⎥ in terms of treated water production ⎣ ⎦

(3)

For each unit of electricity generated nearly, 1.58 kg of CO2, 0.012 kg of SO2 and 0.0046 kg of NO is emitted to the atmosphere from Indian coal based power plants (Dwivedi and Tiwari, 2010; Reddy and Sharon, 2017). Negative impacts caused by SO2 and NO on mankind and crops are very severe compared to the impacts of CO2 (Kalogirou, 2009). Solar energy utilization in Indian diary and paper industries can

6. Environmental benefits and economic analysis of tilted solar distillation unit Enviro-economic analyses are an essential tool to assess the environmental benefits and economics associated with any newly

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mitigate at least 144 and 340 kilotons of CO2 emission/year, respectively (Sharma et al., 2016, 2017). Similarly, utilizing solar distillation unit for wastewater treatment can also mitigate certain amount of CO2, SO2 and NO emissions from conventional fossil fuel based power plants. The net harmful emission mitigated by tilted solar distillation unit is estimated by (Dwivedi and Tiwari, 2010),

Table 5 Energy payback time and emission mitigation potential of tilted solar distillation unit. Wastewater type

⎛⎛ ⎡ Annual energy ⎤ ⎞⎞ ⎜⎜ ⎢ output in terms ⎥ ⎡ Life span ⎤ ⎟⎟ ⎡ Net harmful emission ⎤ = ⎜⎜ ⎢ of treated water ⎥ × ⎢ of the unit ⎥ ⎟⎟ ⎦ ⎢ ⎥ ⎣ mitigated ⎢ ⎥ ⎟⎟ ⎣ ⎦ ⎜⎜ production ⎦ ⎣ ⎠⎟ ⎝ ⎜⎜ ⎟ − [Embodied energy] ⎝ ⎠ Corresponsding ⎤ ⎡ × ⎢ (4) ⎣ emission factor ⎥ ⎦

RO reject Sewage water

Economic analysis is used to estimate the treated water production cost and economic feasibility of tilted solar distillation unit. Treated water production cost per litre is estimated by Kumar and Kurmaji (2013) and Tiwari and Somwanshi (2018),

Total annualized cost TAC = Annual treated water production MY

(5)

Total annualized cost is given by El-Bialy et al. (2016) and Shatat et al. (2013), Fixed annual cost   IR(IR+1)LT ⎞ ⎤ ⎡ ⎛ TAC = CC × LT ⎢ ⎝ (IR+1) −1 ⎠ ⎥ ⎣ ⎦ Annual operation & maintenance cost    LT IR(IR + 1) ⎡ ⎛ ⎞ ⎟⎞ ⎤ + ⎢(AOM factor) ⎜CC × ⎛ LT - 1 ⎥ (IR + 1) ⎝ ⎠⎠⎦ ⎝ ⎣ ⎜

⎟

⎜

⎟

Annual salvage value    IR ⎛ ⎛ ⎞⎞⎤ −⎡ ⎢(SV factor) ⎜UMC × (1+IR)LT - 1 ⎟ ⎥ ⎝ ⎠⎠⎦ ⎝ ⎣ ⎜

Net harmful gas emissions mitigated

(Yr)

CO2 (tons)

SO2 (kg)

NO (kg)

1.70 1.82

23.73 22.05

170.61 158.54

69.68 64.75

lower than the unit distilling RO reject which may be due to former unit’s lower treated water production. Treated water production cost of tilted solar distillation unit distilling RO reject and sewage water considering wick replacement frequency and interest rates is shown in Fig. 8. (Omara et al., 2013) carried out economic analysis by considering jute cloth replacement for every 2.0 Yr. In this study, three cases namely: no wick replacement, replacement once in 5.0 Yr and replacement once in 10.0 Yr are considered for economic analysis. Cost associated with wick replacement is included in capital cost for the respective cases. Treated water production cost increases with increase in interest rate and frequency of wick replacement. Treated water production cost at 5% interest rate is nearly 42.0% lower than treated water production cost at 12% interest rate. Maximum treated water production cost at a 12% interest rate is around 0.040 and 0.045 USD/L for RO reject and sewage water solar distillation, respectively. Minimum treated water production cost at a 5% interest rate is around 0.020 and 0.022 USD/L, for RO reject and sewage water solar distillation, respectively. Treated water production cost in case of 5 Yr once wick replacement, is nearly 12.0% higher than treated water production cost in case of 10 Yr once wick replacement. Treated water cost of fin type solar still (Velmurugan et al., 2008), stepped type solar still (Velmurugan et al., 2009; El-Bialy et al., 2016) and vacuum operated solar basin still (Ibrahim et al., 2017) distilling pretreated industrial effluent and saline water is around 0.034 USD/L, 0.072 USD/L and 0.096 USD/L, respectively. Large scale conventional wastewater treatment unit has a treatment cost of around 0.001 USD/L (Kaur et al., 2012). Treated water production cost of tilted solar distillation unit is lower than other solar stills of smaller capacity but it is higher than conventional wastewater treatment unit. However, treatment cost of solar distillation unit can be further reduced by implementing at large scale (Gude et al., 2010; Tiwari and Somwanshi, 2018).

6.2. Economic analysis

TPC =

Energy payback time

⎟

(6)

Important parameters considered for enviro-economic analyses of tilted solar distillation unit are tabulated in Table 4. Energy payback time and net harmful gas emissions mitigated by tilted solar distillation unit during RO reject and sewage water distillation is tabulated in Table 5. Energy payback time of the unit treating wastewaters is within 2.0 Yr. The unit mitigates significant amount of harmful gas emissions during its life time. CO2, SO2 and NO emissions mitigated by the unit distilling sewage water is nearly 7.0% Table 4 Parameters used for enviro-economic analyses. Parameters for environmental benefit analysis Tilted solar distillation unit embodied energy Clear days (Reddy and Veershetty, 2013) Maintenance days No of operating days Latent heat of evaporation Treated water production* (RO reject) Treated water production* (Sewage water) Life time of distillation unit (Goosen et al., 2000)

Parameters for economic analysis 1397.13 kWh 300 40 days/Yr 260 2372.52 kJ/kg 4.79 L/d 4.48 L/d 20 Yr

Tilted solar distillation unit cost Blended woolen wick cost Usable material cost Interest rate (Kumar and Tiwari, 2009) AOM factor SV factor

1 USD = 66.96 INR. * Based on average daily global horizontal radiation for Chennai- 20.0 MJ/m2-d (Synergyenviron, 2017).

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277.00 USD 16.93 USD 183.47 USD 12% (Public banks); 5% (Private banks) 15% (RO reject); 20% (Sewage water) 20%

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Fig. 8. Treated water production cost of tilted solar distillation unit.

7. Conclusion

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The RO reject and domestic sewage water have been successfully treated in a single step using indigenously developed tilted solar distillation unit. Performance of the unit increases with increase in cumulative solar radiation intensity during RO reject distillation. On contrary, little bit drop in performance of the unit distilling sewage water is noticed during high solar radiation due to clogging of wick caused by dried suspended particles in combination with evolution of non-condensable gases. Continuous wetting of wick by supplying adequate amount of wastewater is highly recommended for achieving enhanced performance of the unit while distilling sewage water. Treated water is odor and bacterial free. BOD and COD removal efficiency of the unit is around 97.3% and 95.8% during sewage water distillation. Heavy metal removal efficiency during RO reject and sewage water solar distillation is in the range of 32.9–82.1% and 51.1–70.6%, respectively. The developed solar distillation unit has huge harmful gas emissions mitigation potential during its life span. Except BOD and COD, other physical parameters and heavy metal concentration of treated water are well within safe drinking water standards specified by Bureau of Indian Standards (BIS, 2012). The treated water obtained by solar distillation of RO reject and sewage water can be used for other domestic activities and irrigation purposes. Effective performance, high treated water quality and low treated water production cost confirms suitability of large scale tilted solar distillation unit for decentralized wastewater treatment in rural and remote areas. Acknowledgement The financial support provided by Department of Science and Technology (DST, Government of India), New Delhi through the research project and Indo-German Centre for Sustainability (IGCS) is duly acknowledged. References Abujazar, M.S.S., Fatihah, S., Ibrahim, I.A., Kabeel, A.E., Sharil, S., 2018. Productivity modeling of a developed inclined stepped solar still system based on actual performance and using a cascaded forward neural network model. J. Clean. Prod. 147–159. Ahmed, M., Shayya, W.H., Hoey, D., Mahendran, A., Morris, R., Al-Handaly, J., 2000. Use of evaporation ponds for brine disposal in desalination plants. Desalination 130, 155–168. Al-Lahham, O., El-Assi, N.M., Fayyad, M., 2007. Translocation of heavy metals to tomato (Solanum lycopersicom L.) fruit irrigated with treated water. Scientia Horiculture

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