The growing human population is exerting incredible stress on natural resources. Clean water, in particular, is an essential resource which is becoming more and more scarce by the year. The thickly populated developing world feels more of the pressure due to limited access to clean water. Erratic rains, aggravated by climate change, and poor storage facilities make things worse for water availability in countries like India. Thus, efforts towards conservation of water are essential for individual livelihoods and national economies alike. Here we report on a novel concept for obtaining significant amounts of pure water by utilizing the condensate generated from domestic and industrial air conditioning units. It is surprising to note that there has been no effort made so far in India and most of the developing world to tap this resource of very good quality water except for few publications in Middle East Asia. Most of this condensate water is allowed to get drained. There are only few reports available from USA where it is suggested that this water can be used for gardening (Aliso Air Inc Report, 2015) . Khan and Zubaidy  have shown how twenty story buildings in Dubai were able to use about 78,000 L of condensate every month reducing the overall 0.5 Kg carbon dioxide emissions per kWh of electricity. Magrini et al.  have shown that in United Arab Emirates, it is possible to harvest water condensate from central air condition systems on a large scale helping conserving precious drinking water resources. Arafat Ali et al.  have tried to quantify the water condensate from air conditioners. They showed that quantity of condensate collected was function of time of operation, temperature difference between outside and inside where air conditioning is working and humidity difference between inside and outside environment. A shopping mall in USA could collect about 875 L of condensate water while a central library building could collect monthly 151,200 L of condensate from central air condition system (Guz  ). Bryant and Ahmed  have shown feasibility of effectively collecting and using the condensate water from central air conditioner in an Institutional system in Doha, Qatar.
In tropical countries, air coolers and air conditioners are used in very large numbers for comfort. Air coolers use water for cooling hot, dry air while air conditioners draw heat from surrounding areas and cool the interiors to desired temperature. One metric tonne air conditioners draw 12,000 BTU (3070 Kcal) heat per hour. Since the relative humidity in many of these tropical and semitropical places is more than 60% for most of the year, air conditioners, while drawing the heat, condense this moisture. The water that drains from a central air conditioning unit or individual unit is condensate that has accumulated on the cooling coils within the unit. Air conditioners pull humidity out of the air and funnel the resulting water, or condensate, into a drain. The quantity of condensate which the air conditioner unit produces varies, increasing as the outdoor humidity level rises. Most modern air conditioners will produce between 15 and 70 litres of water per day depending on the capacity of the unit. A one tonne air conditioner operating for about 7 - 8 hours generates up to 10 L of pure water which if collected aseptically, can be considered as distilled or demineralized water. This depends on the relative humidity. This condensate is typically discarded from the unit through a drain connection or left unattended and gets evaporated in the atmosphere.
We suggest that this water, being generated by condensation of atmospheric moisture, is clean and pure and can be used, if collected and stored properly, for industrial as well as domestic purposes. In this study, we have collected and analyzed condensate from domestic air conditioning units, proving the purity and suitability of the water for domestic such as household and gardening, laboratory and industrial uses such as in batteries and many other applications. It can even be used as drinking water by adding minerals in appropriate quantities. All we would need is to provide a good collection and storage system.
2. Materials and Methods
Sample collection: Condensate was collected from domestic air conditioning units located in Mumbai, Pune Anjar (Gujrat), Kochi, Madurai and New Mumbai. The AC unit was run continuously until 1 L of water was collected in second week of November 2019 in clean sterile mineral water bottles. Samples were analyzed within 24 hours of collection. They were stored in refrigerator at 8˚C before analysis.
Water analysis: Microbiological and chemical analysis of the water was performed at National Accreditation Board for Testing and Calibration recognized Laboratories (NABL) viz. (Vipanan Laboratory, Pune, Universal Purity Testing Laboratory, Borivli, Mumbai and Tamil Nadu Water Supply and Drainage Board District Water Testing Laboratory, Madurai. Welpsun Laboratory, Anjar is an inhouse testing facility for Welspun cloth mill recognized by Gujrat Pollution Control Board.) Specifically, the water quality was measured in terms of pH, total dissolved solids (TDS), organic carbon (T. O. C) contents, electrical conductivity, hardness, E. coli count and total coliform count. All the analytical methods are listed in Table 1 and approved by NABL, India except for total organic carbon (TOC) analysis. Though TOC analysis does not figure in IS (Indian Standard) 10,500 protocol  for drinking water testing, it was included here considering the overall significance of this project. TOC may be considered as an
Table 1. Analytical procedures for various parameters in NABL accreditated labs.
indirect fast indicator of Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). Given that the condensate formation and its outflow are almost instantaneous, it is unlikely that there would be time for dissolution of any organics unless the tube through which it flows out is highly contaminated with biological growth. Table 1 lists the analytical parameters used for AC water testing and their maximum permissible limits.
3. Results and Discussion
Air conditioner water condensate from individual window or split air conditioners were collected from various cities in India. Table 2 shows the meterological data collected from official internet portals with respect to average relative humidity round the year in various cities in India. These figures might vary to some extent but can give us general idea about their potential. These cities account for more than 70% of the country’s air conditioners. Especially the coastal cities like Mumbai, Panjim, Kochi, Bhavnagar, Chennai, Vijag, Cuttack, Bhubaneshwar, Kolkata and others (Marked yellow) where relative humidity is more than 60% throughout the year have a great potential for use of air conditioning systems. A huge water resource can be collected from these systems.
These cities with some other metro cities in Central India like Bengaluru, Hyderabad, Jaipur, Indore, Bhopal, Lucknow, Kanpur, Ahmedabad, Vadodara, Gwalior, Kanpur and smaller citied in Punjab, Madhya Pradesh, Uttar Pradesh, Jharkhand, Uttarakhand, Maharashtra, Gujrat and Karnataka contribute to more than 35% of Indian population. These cities are growing and are becoming hubs for many industrial and software activities resulting in ever increasing demands for air conditioning. The use of air conditioners has gone up considerably in last decade. The information technology sector and tourism have contributed for increased demand for the air conditioners across the country.
There are several private and government sectors where central air conditioning systems are used for 24 hours a day and 365 days a year. These point sources will contribute to recovery of very large quantities of good quality water. The air conditioned buses, cars, trucks and trains are another important sources for condensate water. While large scale collection and use of such a decentralized resource will almost certainly not be economical, the use of this water at
Table 2. Meteorological data on average relative humidity in important Indian Cities round the year (average of last ten years 2009-2019).
source for domestic and industrial purposes is easily possible. Large industrial and building-scale central HVAC systems generate much larger amounts of condensate, which can then be collected and used at source as appropriate. In many such places, air conditioner condensate is collected centrally only to be drained off! The present study hopefully will help in changing the mindset and systems will be evolved to collect this condensate and use it effectively.
Table 3 shows the results of analysis of water condensate samples collected from various locations in India during the month of November 2019. The theoretical collection rate as calculated using basic thermodynamic principles is included for reference. It is clear that in most tropical climates, where relative humidity is often above 60%, a daily collection of up to 10 L liters of water is feasible from a single air conditioning unit of 1 MT capacity and up to 25 - 30 L for 2 MT air conditioner unit. We have actually collected 40 - 50 L water condensate from a location in Vashi in 6 hours (New Mumbai) where 5 air conditioners of 1 MT capacity each are connected on line.
Analysis of the water quality of samples generated from domestic air conditioners is summarized in Table 3. Microbiologically all the samples were bacteria free except the sample in Kochi where it was collected from an open tank which collects air conditioner condensate centrally. Since it is an open tank, the aerial bacterial contamination is not unexpected. It is encouraging to note that none of the samples showed presence of E. coli which is indicator of water quality for potability. The chemical parameters like TDS, organic carbon and hardness also showed that the air conditioner condensate is extremely pure and might be comparable with distilled or demineralized water. The maximum permissible limit for TDS in drinking water as per Indian standards is 500 mg/L while the results show that it varies from 10 to 91 mg/L in these condensates. The values obtained for hardness quality also are indicative of high purity of these
Table 3. Analysis of quality of AC condensate water (1 MT unit) collected from different locations in November 2019.
Sample was taken from open tank where AC condensate is collected centrally.
condensates. If a good closed loop collection system is developed, these levels can be brought to almost zero. Whatever small impurities reflected in TDS and the observed conductivity values were mainly due to contamination during collection. Since none of the air conditioner unit in India is designed to collect this condensate water aseptically and almost all of this condensate is flowing out into environment, it is expected that the drain end of the unit will not be maintained specifically for collection purpose. If the air conditioner manufacturers take up this issue seriously and design a small tank of maximum daily output capacity, it would boost the use of this very pure resource which is getting wasted.
There are few reports about fungal contaminations and bacterial infections due to their growth near outlets. The bacterium Legionella pneumophila is responsible for most cases of Legionnaires' disease. Outdoors, legionella bacteria survive in soil and water, but rarely cause infections. However, legionella bacteria can multiply in water systems made by humans, such as air conditioners . In a study published in 2005, a good number of hospital infection cases were linked to contamination from AC units in India . These possibilities can be eliminated if a good collection and storage system is provided along with the air conditioning unit.
Air conditioning units generate significant amounts of chemically and microbiologically pure water during their normal operations. This study shows the feasibility of collecting and using this water at source for both domestic and industrial purposes. Large industrial AC systems, trains and cars can also potentially bottle the condensate and convert a waste stream into a revenue source. Current AC unit designs include a drainage tube to remove the condensate. A simple water collection system can be designed and included in the unit at the time of manufacture in order to facilitate its use. There are roughly about 6 million air conditioner units active in Indian houses and establishments and the number is rising exponentially. The high purity water condensate collected from all these units will be between 50 and 100 million litres a day for most part of the year. We can no more ignore this issue and allow this precious resource to get drained. An aesthetically good looking storage tank near the air conditioner unit inside the hall would maintain the purity of this condensate and high value applications like battery or drinking water. It would certainly encourage the people to collect this condensate.
As the dual challenges of population growth and climate change stress the planet’s resources and threaten livelihoods, simple, decentralized technologies will form a major part of the solution to the problem of water scarcity. Even as we collectively focus scientific and engineering resources on big, revolutionary technologies to mitigate our challenges, it is valuable to invest in easily executable technologies that can have a meaningful impact on the way the general public gets involved in solving these problems. In addition to the obvious impact on reducing water waste, the sociological impact of this technology will also be significant, as it will encourage more people to focus on the ideals of conservation and minimizing our environmental footprint. Ultimately, simple technologies with widespread adaptation will have the most impact on our collective efforts to survive on a crowded and climatically changing planet.
It is high time that leading manufacturers of air conditioners like Voltas, BPL, LG, Hitachi, Godrej and many others take cognizance of this study not only in India but worldwide and help in developing a good collection system for water condensate. This would also offer good business opportunities.
The authors express their deep gratitude to Dr. Ramesh Kumar (Welspun, Anjar) in collecting and analyzing samples. We are thankful to Dr. Shubhada Nayak (Vice Principal, KBP College, Vashi), Dr. Anu Appukuttam (Kochi Refineries), Dr. Varsha Kelkar, Mumbai University, Dr. Sukruta Pethe and Ms. Varsha Bapat, Andheri, Mumbai, Mr. G. T. Karnani Bandra, Shri Avinash Gavankar, Goregaon, Mumbai and Mr. Sivakumar (Madurai) for their help in collecting samples in Anjar, Vashi, Kochi, Mumbai and Madurai.
 Khan, S.A. and Al Zubaidy, S.N. (2013) Conservation of Potable Water Using Chilled Water Condensate from Air Conditioning Machines in Hot and Humid Climate. International Journal of Engineering & Technology, 3, 183-188.
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 Legeionnair’s Disease (2019)