"Technology is key and the backbone of any water, wastewater and sludge project"

"Technology is key and the backbone of any water, wastewater and sludge  project"

Dr. Indra Mitra is Director (Technology and Project Development) for Cambi, India. Dr. Mitra is a Business Professional and a Technologist and established two successful start-ups (foreign technology companies) in India. He is an International Expert in Water, Wastewater & Sludge (WWS) Treatment with 25 years of experience in the USA and India in Technology & Process, Policies and Regulations and Leadership Management. Dr. Mitra was recognized as a “Who’s Who” in Water & Wastewater by the American Academy of Environmental Engineers and was a registered Professional Engineer (PE) of the State of Virginia, USA.

He was awarded the “Man of Excellence Award, 2021” for Environment and Water in India. He was recognized as a Water Leader by leading water organizations in India in 2022 and 2019. In 2018, he was part of the India Business Delegation for Water Industry to Italy. In 2022, he was part of Norway Business Delegation to Gujarat. Currently, he is part of the Expert Team of JICA (Japan International Cooperation Agency) as Sludge Expert to develop a Guidance Document for the Management and Disposal of Sewage Sludge in India for NRCD (National River Conservation Directorate) under the Government of India (GoI). He was invited to join the Expert Panel for cGanga (Centre for Ganga River Basin Management and Studies) under GoI.  

Dr. Mitra is a world-renowned expert technologist in Biological Treatment, Physical-Chemical Treatment and sludge treatment of Municipal and Industrial wastewater. He has worked on hundreds of Wastewater Treatment Plant (WWTP) projects funded by US EPA (Environmental Protection Agency) and International Funding Agencies (JICA, World Bank, ADB (Asian Development Bank), EIB (European Investment Bank)), Municipalities, States, Government & Industry in USA and India.

For the last 14 years, Dr. Mitra has worked in Leadership Positions as Business Head/VP/Director. He has worked for Cambi (Europe), World Water Works (USA), NJS Consultants (Japan), Toshiba Water (Japan) Jacobs (USA), Tetra Tech (USA), etc. Dr. Mitra has completed B.Tech from IIT, Kharagpur, India, MS from Vanderbilt University, USA, Ph.D. from Lehigh University, USA & MBA from Virginia Tech, USA. He is an invited speaker for numerous conferences, seminars and workshops. He has authored several books and papers and is a member of several leading associations and organizations.


As a key player in the water industry how has your journey been in the field of work – water, wastewater and sludge?


My journey in WWS field has been extremely rewarding. I have been able to contribute significantly to my field both in US and India. My passion for Sales and Technology, thirst for knowledge and quest for learning made my work very enjoyable. In my leisure time, I am busy reading articles and books on WWS. I was fortunate to work with top global companies and experts for each technology and learnt and mastered each of the technologies.  WWS is a highly technical field and unless one has good technology know-how one cannot win projects. Sales with the right price can only come from a solid technical proposal.

I was fortunate to have vast exposure to the market globally. In the first 10 years of my career, I worked mostly with US EPA developing wastewater regulations, policies, standards and guidelines for US industries. After spending 17 years in the USA, I returned to India. I used my strong technical background to win numerous projects in industrial and municipal spaces easily. For the last 25 years, I have worked on hundreds of WTPs (Water Treatment Plants), WWTPs & Sludge Treatment projects. Work has been very satisfying and fulfilling. I have established two start-ups in India and have been recognized as a Water Leader both in India and globally.

Academically, it was very rewarding and satisfying. I graduated from top institutes in US and India and was able to complete Ph.D. with a 4.0/4.0 GPA. I am an invited speaker for numerous conferences, seminars and workshops and have authored several books and papers.

For the last few years, I get my satisfaction in giving back to Society through knowledge sharing, teaching, advising, writing papers and books. Among other initiatives, lately, I have been a key contributor to initiating and accelerating the requirement of sludge treatment in India to protect the health of citizens.


In your opinion, what does the Industrial wastewater landscape look like in India, and what are the growth challenges in this sector?


India's WWTP market stood at USD 2.4 billion in 2019 and is projected to reach USD 4.3 billion by 2025 (Business Standard, 2021). Although the Industrial wastewater market is growing well in India, it may require a slight change in trajectory for the betterment of the environment. Most important is to focus on technology to make cost-efficient and high performance ETPs (effluent treatment plants) and ZLD (zero liquid discharge) plants to protect the environment holistically. With rapid economic growth, increasing population and rising urbanization, the industrial wastewater sector is growing fast. Challenges faced mostly are related to too much importance to cost in awarding tender (L1), inaccurate raw wastewater characteristics used for design, improper selection of technology and design, low performance, absence of piloting and poor quality of treated effluent. Besides cost, more focus on a selection of sustainable and environment-friendly technology, accurate design, performance, energy minimization, decarbonization, resource optimization, quality and training of operators and designers needs to be given. The good news is that many companies are moving in that direction.  

Technology is key and the backbone of any WWS project. Like developed countries, we need to develop technology specialists for each technology. In the medical field, we have General Physician, but also Heart Specialist, Eye Specialist, etc. Similarly, in the WWS field, besides General Experts we need Specialists in each technology – such as Biological Treatment Specialist, MBBR Specialist, Anaerobic Digestion Specialist, etc. Each Technology Specialist should have all the latest knowledge of the technology. This will save the industry a lot of money by building treatment plants with appropriate technology and the right design. If you see all top WWS companies globally are very strong in technology. Recommend that companies develop a technology career field in all EPC and Consulting companies.

India needs at least a total of 3,000 – 4,000 STPs to treat 100% sewage in urban areas and a total of about 5,500 – 7,000 STPs to treat sewage for the entire country.

What are your recommendations for some of the commonly used technologies, standards and management of wastewater in industrial sector.


Some of the recommendations for industrial sector are provided below:

  1. ZLD using RO (reverse osmosis) membranes is a good concept for recycling and reuse of wastewater. However, if you look at it holistically, it is not the right solution for environment because of the following reasons:

    • Rejects are generated during RO process which are hard to dispose. So, in an RO process one has converted a relatively clean harmless wastewater to a problematic reject which is a threat to environment.

    • RO process consumes a lot of energy and chemicals which are a threat to environment. Evaporators consumes huge energy and not environment friendly.

    • RO membrane life is 1-3 years. Disposal of these membranes is a problem and presents an environmental hazard.

    • RO process and evaporators are expensive. 

      So, use of RO must be discouraged and minimized. It should be kept as the last option. Properly treated wastewater with filters (without membranes) meet the recycle water standards in most cases. This treated wastewater can be used for gardening, toilet flushing, cleaning, irrigation etc. RO is required only when very high quality of water is required (such as cooling tower make-up). So, percentage of treated wastewater that needs RO treatment may be very low.

  2. Many permits require a discharge limit of 2,100 mg/L total dissolved solids (TDS). This limit needs to be revisited. 

  3.  All industrial ETPs must do a pilot before going full-scale. This will reduce risks and save huge costs.

  4.  Currently, organic sludge in many ETPs and common effluent treatment plants (CETPs) are not treated in anaerobic digesters. They are disposed off in landfills and presents environment hazards and high costs. Anaerobic digesters will generate power, stabilize sludge and reduce the volume of sludge to disposed off. Advanced Anaerobic Digestion (AAD) will further improve the process and enhance benefits. Anaerobic digestion of organic sludge must be made mandatory to protect environment, save landfill space, recover energy and mitigate carbon emissions. Smaller ETPs can send sludge to a centralized sludge centre where the combined sludge can be digested before disposal.

  5. More trainings, technical workshops, technical conferences focussing on technology, treatment, performance and case studies must be done for sharing information and knowledge. Government and Industry bodies should set up a technical team and come up with treatment technologies for various wastewater streams along with case studies. This information should be shared free on web for others to learn and reference. The website should also contain Fact Sheet of various technologies and application.


Is India planning and investing considerably well in sewage wastewater treatment facilities considering the amount of sewage generated in urban and rural centres?


In last 7 years India has invested well in STPs (sewage treatment plants) but much more needs to be done as we have miles to go. NMCG (National Mission for Clean Ganga) is investing heavily in STPs in Ganga basin but the investment needs to be done all over India. Recently, BMC (Brihanmumbai Municipal Corporation) awarded contracts to build/upgrade 7 large STPs in Mumbai worth more than Rs 26,000 crores (Governancenow.com, 2022)

Currently, we have only 1,600 STPs in India – about 50% of which are built in the last 7 years. With a population of about 1.4 billion, it is estimated that India generates more than 150,000 MLD (million litres per day) of sewage of which only about 72,300 MLD is collected in urban areas. Presently, the existing treatment capacity is 36,600 MLD (including the proposed STPs). This leaves a treatment gap of about 40,600 MLD, which is 56% of sewage collected in urban areas (an improvement from 62% untreated sewage in 2015) (CPCB, 2021). So, India needs to invest heavily on building sewerage and STPs to protect the environment and health of the citizens. At a minimum, India needs to build STPs to treat 100% of sewage generated in urban areas.

Currently India with a population of 1.4 billion has more than 1,600 STPs. Compare that with the USA which has 16,000 STPs for a population of 331 million. Based on the current population, I estimate that India needs at least a total of 3,000 – 4,000 STPs (depending on capacity) to treat 100% sewage in urban areas and a total of about 5,500 – 7,000 STPs (depending on capacity) to treat sewage for the entire country. This number is going to increase with population growth, more urbanization, lifestyle change and as more sewerage pipelines are laid. So, huge investment is required in this sector for the next 5-10 years.

If untreated sewage is discharged into rivers, lakes, etc then it will spread diseases, kill aquatic flora and fauna and present a nasty smell and an ugly scene. So, sewage is treated in STPs to purify the wastewater before discharge into the waters of India. During this sewage treatment sludge is generated in STPs. Untreated sludge, like untreated sewage, also presents a health and environment hazard. By treating sewage, we have done only 50% of the work and the other 50% is done when sludge is treated to meet hygienic standards. So, sludge produced in STPs needs to be properly treated, stabilized and hygienized before disposal.


In your opinion are municipalities serious about STP discharge standards?


In 2019, NGT issued new sewage discharge standards in April 2019, and were upheld by the Supreme Court of India in June 2021 (Cambi, 2021). The discharge standards are at par or even more strict than in many developing countries. All new and existing STPs are required to upgrade their plants to meet the new limits. As we aspire to become a developed country, we should make sure that the environment and health of the citizens are protected. The health, social and environmental costs of a country run into several billions of dollars. It is the duty of the governments and municipalities to ensure that the health and environment are protected. Discharge of dirty/untreated sewage and unhygienized sludge into the environment causes diseases, destroys aquatic flora and fauna and destroys the environment. So, it is mandatory and cheaper to build STPs with proper sludge treatment when compared to the health, social and environmental costs of India and its citizens.

Recently, NGT has imposed hefty fines running into thousands of crores on several states for discharging untreated/partially treated solid and liquid wastes into the environment. The citizens have also become very conscious and demand a clean environment. With the coming of remote monitoring, artificial intelligence and other advanced techniques, it is now easy to monitor effluent discharge from STPs even sitting thousands of miles away. Since the NGT limits are applicable with immediate effect, it is high time that the municipalities become serious about meeting the NGT guidelines from now on otherwise hefty NGT and other fines will be levied.

Companies must develop a technology career field in all EPC and consulting companies

What more needs to be done to promote sewage water treatment and reuse?


Generally, the economy comes before health and environment in priority in most countries. As India is becoming rich and a developed country, the need and demand for a better and cleaner environment is rising, particularly in urban and semi-urban areas. There is an increasing interest in sewage treatment and reuse. India built nearly 800 STPs in the last 7 years, thereby doubling the no of STPs. The percentage of untreated sewage has decreased from 62% in 2015 to 56%. However, as discussed above, more needs to be done.

NGT (National Green Tribunal) has set new effluent standards to which all STPs need to abide. This means, in addition to new STPs, all existing STPs need to be upgraded to meet the new standards. The new standards are very strict and the treated wastewater will be of very high quality which can be recycled and reused with no or little treatment. India is a water-stressed country supporting 17% of the world’s population with only 4% of the world’s water resources. Studies show that the projected per capita water availability will become 1,401 m3 and 1,191 m3 by 2025 and 2050 respectively and eventually India is likely to become a water-scarce country unless corrective measures are not taken today. So, every effort should be made to recycle and reuse high-quality treated sewage. Some municipalities, such as Surat, Mumbai, Nagpur, Pimpri Chinchwad, etc. are taking initiatives to recycle and reuse treated sewage. Many industries, and industrial clusters are interested in using treated wastewater. Tremendous investment and work need to be done in this sector. Fortunately, the economics works in favour of recycling and reuse of treated wastewater particularly when selling to Industries. Industries pay about Rs 30-50 /m3 for water intake whereas operating costs of STPs are only Rs 3-8/m3 and the capital costs are Rs 1.5-3 crore/MLD. Of course, one needs to add the cost of transportation of treated wastewater from STPs to the Industry site. I have done some evaluation for some STPs in India under World Bank Project and found the economics are favourable. Presently, numerous industries are recycling and reusing treated wastewater including from STPs.

One way to accelerate STP building is privatising the STP. The UK is a good example. UK model needs to be studied for application in large Indian cities as pilots and see how it can be modified for the Indian scenario. Privatization will also ensure bringing in the latest technologies which are most cost-efficient, energy-efficient and environment friendly. This will reduce the capital and operation costs of STPs drastically.

In addition, the following needs to be done immediately to reduce the cost to build STPs:

a.      Every effort should be made to recycle and reuse water. Municipalities can sell recycled water to industries and industrial clusters to generate revenue.

b.     Sludge should be treated to produce biogas (renewable energy) and to produce fertilizers/soil amendments that can be sold. Washington DC STP (DC Water) generates power from sludge to reduce electricity bills in STP and earns revenue by selling treated sludge as fertilizer. This model needs to be studied and see how it can be reproduced in India.

In addition, the following may be pursued:

a.  Every effort should be made to recycle and reuse water. Municipalities can sell recycle water to industries and industrial clusters to generate revenue.

b.   Sludge should be treated to produce biogas (renewable energy) and to produce fertilizers/soil amendments that can be sold. Washington DC STP (DC Water) generates power from sludge to reduce electricity bills in STP and earns revenue by selling treated sludge as fertilizer. This model needs to be studied and see how it can be reproduced in India.

c.   Each city should build centralized sludge centers to collect and treat sludge from small and big STPs. I envision, broadly, that all cities and towns in India have a minimum of 4 sludge centres – East, West, North and South. These sludge centres will process sludge and will do co-digestion with food waste also.

We all know that water demand in India is huge and the treated sewage can be reused in irrigation, cleaning, industries, etc. India needs to develop standards and regulations for the recycling and reuse of wastewater for various uses.  I worked as an expert for a World Bank project where we outlined the standards for recycle and reuse of treated wastewater (World Bank, 2022). This can be used as a start to develop standards for recycling and reuse.

In my opinion, a lot of work needs to be done for sewage treatment, recycling and reuse of treated wastewater and sludge treatment and reuse promoting a circular economy using sustainable and environment friendly solutions.


Where does India stand in terms of wastewater technologies in sewage treatment as compared with other countries? How much has technology advanced in sewage treatment in the past three years in India?


Technology is generally driven by the regulations and effluent standards set by the country. In 2019, NGT issued effluent standards for STP which is at par or even more strict than many developed countries. So, new technology companies are entering India. Since the new regulations are only 3 years old, India needs to do a little catching up in sewage treatment technologies to be at par with Western countries. It is just the beginning. To meet the total nitrogen (TN) limit of 10 mg/L and total phosphorus (TP) limit of 1 mg/L, numerous technologies are available globally. I believe more than 80% of the STPs globally use some variation of activated sludge (AS) process such as Modified Ludzack-Ettinger (MLE), anaerobic-anoxic-aerobic (A2O), Bardenpho process, etc. In India sequential batch reactor (SBR) technology is widely used.

We need to look at technologies holistically and not just look for the cheapest option. Experience has shown that the so-called “cheapest solution” often becomes the “most expensive” solution in the long run. Performance, quality, experience and plant references of EPC and Technology companies are very important and should be evaluated closely. We need to be open to advanced technologies and evaluate the technologies holistically and on merit. Not all size fits all. The good news is that the trend is changing and the municipalities are welcoming new technologies. Last 3 years after the strict limits imposed by NGT many advanced technology companies have entered India in the STP area. In recent years, sludge treatment has also become important with tenders requiring sludge be treated to make it free of pathogens by meeting US EPA Class A standards. As a result, many Class A technology players have entered India.

There are many advanced technologies yet to be installed in India. These include side stream treatment such as Annamox/Demon, Phosphorus recovery systems, CambiTHP (Cambi Thermal Hydrolysis Process), Nereda and numerous other technologies. Globally, there has been a push to recover energy from STPs as much as possible and make STPs a zero energy plant.

In order to encourage foreign advanced technologies, cGanga has started ETV (Environmental Technology Verification Program) which enables foreign technologies to enter India. We are happy to inform that CambiTHP has been selected in the ETV program.

The knowledge level to thoroughly understand technologies need to be improved and a huge capacity building needs to be done for different technologies. It will take some time to build up the knowledge base among industries as the advanced effluent limit in India was issued just three years ago. Compare that with Western countries, which had advanced limits issued 20 or 30 years ago and have built a huge knowledge base over the years. So, it is very important that the technical expert who must be appointed to evaluate different technologies in India should be technically sound and preferably has exposure globally. Since wastewater is a highly technical field, the appointment of a sound technical person will save project costs. Proper training and workshops are required on a regular basis to train the operators, designers and decision-makers.

As mentioned earlier, technology is key and the backbone of any WWS project. We need to develop technology specialists for each technology. Each technology specialist should have the latest knowledge of the technology. This will save the industry a lot of money by designing the right STPs. My recommendations are that companies must develop a technology career field in all EPC and consulting companies.

Proper evaluation of various technologies and selection of the right technology is key. Technologies must not be evaluated only on costs, especially, capital costs. It should be evaluated on a holistic basis – including life cycle costs (CAPEX + OPEX) over long periods (20-30 years), land costs, environmental and social impacts, compactness, energy, odour, noise, aesthetics and carbon footprint. There should be an evaluation matrix with points for each item listed above. Tenders should address the above items to install sustainable advanced technologies. As STPs run for 40-50 years or more, it is important to give preference to technologies with lower opex. Most modern and successful technologies have low operation costs. This will enable municipalities to save costs over time.  This will enable more modern and advanced technologies to enter the Indian market.

Many Western utilities have gone through a period of trying times, failures, and lessons learned. Indian entities (utilities, consultants, contractors, decision makers) must establish constant interaction with international entities (utilities, consultants, experts) so that Indian entities can learn from the experience of International entities so that the same mistakes are not repeated in India. There is no point in re-inventing the wheel.

In today’s world, sludge is no longer considered a waste, but a resource that can be used to generate energy using anaerobic digestion

What kind of challenges do you face while treating sewage water?


With proper technical know-how, sewage treatment is in fact one of the easiest wastewater to treat. Also, a huge amount of knowledge and experience is available globally for sewage treatment. So, sewage treatment for Western countries for a typical STP is not a big challenge except for special cases.

However, many STPs in India do not meet the effluent limit. One of the main reasons is that the STPs are overdesigned. Since the sewage flow and loads are estimated by taking into account a 20-40 year time horizon, the design flow and the raw sewage BOD concentration/load are generally far above the actuals. This results in the overdesign of the system resulting in higher STP costs and poor plant performance. Recommend that based on current and future loads, the design uses a modular approach with multiple treatment trains. It is recommended that the STPs be first designed based on current/actual BOD loads with some safety factor (say 25-30%). Additional land be budgeted for future expansion. Later, additional treatment trains can be installed over time when additional BOD loads are received or expected to be received. This will save both capital and operating costs.

Another problem in India is that many STPs operate with very high SRT (solids retention time) mainly because of overdesign as discussed above. This results in poor performance and high energy consumption. For a typical STP in India, the SRT should not be more than 6-10 days under operating conditions to meet the current discharge limit of TN of 10 mg/L. Higher SRT is not desirable as it aerobically digests the sludge in biological tanks resulting in higher oxygen (energy) consumption and destroying the VS (volatile solids) which could otherwise be used to generate biogas in anaerobic digesters. Higher SRT also affects plant performance and the settling of sludge.

Another challenge that India faces is that in many places the sewage is dilute and the BOD concentrations are low. Adequate BOD is required to do denitrification to achieve the effluent limit of TN of 10 mg/L One option is to bypass the primary clarifiers or not to build primary clarifiers. In both cases, the sludge generated will consist of only WAS (waste activated sludge). The properties of WAS are very different from those of primary sludge. WAS is extremely difficult to destroy in anaerobic digesters to generate biogas. The VS destruction of WAS in conventional digesters varies between 20-35%.  Also, for sludge with only WAS, it is very difficult to achieve 25% dewatered solids as required in many tenders. The dewatered solids vary between 18-22% solids. The US EPA regulations require 38% VS destruction to meet the vector attraction reduction requirement for land application and landfilling. So, in such cases, THP (thermal hydrolysis process), a state-of-the-art technology in sludge treatment, maybe the preferred choice as it disintegrates WAS and makes it biodegradable. Digesters with THP achieve 50% VS destruction with WAS and the dewatered sludge can achieve 28-30% solids. In addition, THP produces pathogen (disease causing organisms) free Class A sludge with significantly reduced odour and reduces digester volume 60-70% with 20-50% increase in biogas production. 

As mentioned earlier, knowledge level of technologies needs to be improved and a huge capacity building needs to be done. Proper training and workshops are required on a regular basis to train the operators, designers and decision makers.

DBO (design build, operate) Projects awarded in India are based on lowest cost (L1). This presents a huge obstacle to quality and advanced technologies. Project evaluation must be done based on various other factors besides cost. An evaluation matrix may be developed which will include life cycle costs, land area, performance, qualifications of professionals working, carbon emissions, environmental benefits, risks, quality, aesthetics and other factors. For each criterion, marks may be assigned and the project with the highest score wins. Since STP is run for 40-50 years, lifecycle cost should be based on 20-25 years. It is to be noted that, unlike DBO contracts, many consulting projects are awarded based on 80% technical and 20% costs.

As STPs run for 40-50 years or more, it is important to give preference to technologies with lower opex

How important is sludge treatment process? With regards to sludge what are the norms to manage, treat or dispose it?


Sludge is produced during sewage treatment in STPs. Untreated sludge spread diseases and present a nasty smell and an ugly scene. In addition, if sludge is discharged to rivers, lakes, etc. then, like untreated sewage, it may also kill aquatic flora and fauna. So, sludge must be properly treated and disposed. Untreated sludge, like untreated sewage, presents a health and environmental hazard.

Management and disposal of sludge generated from STPs present a huge challenge. Sludge must be properly treated before disposal. Treated sludge is called biosolids. EPA, UK, European Union and many other countries have developed standards and regulations for sewage sludge disposal. EPA regulation on STP sludge is popularly known as the EPA Biosolids Rule. It is most widely used globally and the standards are comparable with those used for other countries. Recent tenders in many cities in India such as Mumbai, Delhi, Chandigarh, Ahmedabad, etc., have required EPA Class A standards for disposal of treated sludge. Class A standard ensures that the sludge is pathogen free and so safe to handle. We need to develop sludge regulations for India as soon as possible. 

As per EPA Biosolids Rule, disposal routes of treated sludge (biosolids) from STPs are:

(i)             Incineration,

(ii)            Surface disposal such as landfill/monofill

(iii)          Land application – treated sludge is used as fertilizer/soil amendment/conditioner. 

EPA Biosolids Rule has established criteria for each of the disposal options which must be met by the treated sludge before disposed. Incineration and landfill are not sustainable solutions. Incineration pollutes the air and creates foul odour, and often leads to large-scale protests by the local population to close down the Incinerator. Cleaning of flue gases from incineration is required, which adds to the cost. Also, the ash created from Incineration needs to be sent to landfill which takes up land. Sending sludge directly from STP to landfill takes up lots of land, a scarce resource in India, and contributes to greenhouse gas emissions.  Also, the landfills, if not properly maintained (as most landfills in India), are a breeding ground for diseases. So, neither incineration nor dumping in landfills are sustainable solutions. 

The most sustainable solution promoting circular economy is to treat the sludge properly and land application - use it as a soil conditioner/amendment or fertilizer that can be applied in gardens, parks, lawns, playgrounds, land embankments, forests, agriculture etc. However, currently, the sludge produced by STPs is not adequately treated and contains pathogens.  This exposes the users to health hazard. The sludge must be treated to produce pathogen-free biosolids (EPA Class A standard) and be stabilized to reduce odour and vector attraction before it can be land applied. Pathogen-free Class A biosolids can be easily achieved by treating the sludge with a Class A technology such as CambiTHP.


What is your recommendation to treat sewage sludge?


Sludge produced in STPs needs to be properly treated, stabilized and hygienize before disposal. STP without sludge treatment is only half the work done as diseases may spread through untreated and unhygienic sludge. Unfortunately, sludge treatment has been totally neglected in India. More than 80-90% STPs in India have no sludge treatment. These STPs only have thickening and dewatering to reduce the volume of sludge to be hauled. They do not have digesters. Dewatered sludge with no treatment presents a health hazard. Dewatered sludge with no treatment can only be landfilled/incinerated following proper guidelines and procedures. As India aspires to be a developed country, proper treatment of sludge is required to protect the health of the Nation.

At a minimum, sludge must be anaerobically digested (or similar treatment) to meet Class B standards (partially hygienize and stabilized) as per EPA Biosolids Rule to protect the health of the citizens. This is practiced in many developed countries. Recently, there is a push globally and in India for meeting Class A standards (fully hygienize and stabilized) as per EPA Biosolids Rule. This enables the Class A sludge to be used as a fertilizer/ soil conditioner promoting circular economy. Health of the soil in India is not good and use of Class A sludge will improve the soil conditions and will also save landfill space. Class A sludge is considered pathogen free and protects the health of the citizens. Clean Ganga and NMCG is also working on sludge treatment and may come up with a policy or guidance for the same.

In my opinion, treated sludge can be disposed of in the following ways (in order of preference):

1.     Sludge reuse: Sludge can be used in cement (e.g., as a binder) and other industries.

2.     Land application: Sludge is used as fertilizer/soil amendment/conditioner.

3.     Thermal Process: Used as fuel, generate byproducts, etc. Examples include incineration, pyrolysis, gasification etc.

4.     Landfill

It is important that a sustainable method is adapted for sludge management. This may require one or more  disposal options. For example, in one project in China, the sludge used to be sent to a thermal plant as a fuel. But after a few years the thermal plant was shut down and the STP had a hard time to find an alternative disposal method. So, for a sustainable solution, a default disposal option should be made available as back-up. Land application should be considered as the default disposal option as all sludge if properly treated can easily be land applied. So, all sludge, irrespective of their final disposal option, must be treated to achieve a minimum requirement of land application except the heavy metals criteria. Land application requires pathogen reduction (Class A or B) and vector attraction reduction. Heavy metals criteria as per EPA Biosolids Rule is applicable only when the sludge is actually land applied.

My recommendation is that all sludges in STP, at a minimum, must meet land application requirement of Class A requirement, stabilized and capture biogas which can be used as renewable energy or flared. The minimum standard for the treatment of sludge is based on the following facts:

1.     Whatever disposal method one chooses, the sludge is likely to come in contact with humans. So, pathogen-free sludge (Class A) is required to prevent spread of diseases and to protect health.

2.     It is very difficult to track sludge after it leaves the boundaries of STP. So, to reduce vector attraction and spread of diseases the sludge must be stabilized (achieve 38% volatile solids destruction or by other methods)

3.     Sludge is a resource and can produce biogas (renewable energy). Biogas, if not captured, will contribute to greenhouse gas emissions. On the other hand, captured biogas can be used as renewable energy promoting a circular economy or flared.

In today’s world, sludge is no longer considered a waste but a resource that can be used to generate energy (renewable energy) using anaerobic digestion.  Also, properly treated sludge can be used as fertilizer/soil amendment/conditioner, enhancer, etc. The health of the soil in India is extremely bad mainly due to extensive use of inorganic fertilizers. Treated sludge is a great source of carbon which can be used to improve the health of the soil. Properly treated sludge can be used by municipalities as fertilizer/soil conditioner for parks, playgrounds, forests, land embankments, etc. which will save some or all of the fertilizer costs that the municipality buys every year. 

India has numerous small STPs and it may not be economical to treat sludge at individual STPs. For that, a centralized sludge centre may be established which will process sludge from multiple STPs. This will reduce cost drastically. This sludge centre model has been successful all over the world. I envision that all cities and towns in India have a minimum of 4 sludge centres – East, West, North and South. This sludge centres will process sludge and can do co-digestion with food waste also.

Most modern and successful technologies have low operation costs. This will enable municipalities to save costs over time.  This will enable more modern and advanced technologies to enter the Indian market.

What is the approach of Cambi India to contribute towards global sustainability?


CambiTHP is a sustainable, green and environment-friendly technology that fulfils 8 of the 17 Sustainable Development Goals (SDGs) of the United Nations (UN). CambiTHP promotes circular economy by converting waste to wealth and recovering and recycling resources from waste – Fertilizer and renewable energy (bioenergy).  As a result, many STPs around the world have started renaming their facility as Water Resource and Recovery Plants.  In some cities, buses are run from the methane produced by the CambiTHP biogas plants.

Besides, installation of CambiTHP technology will have the following advantages:

  • Reduces the volume of the anaerobic digester by 60-70%. This will save huge costs and footprint and carbon footprint.

  • Increases the volatile solids destruction in anaerobic digesters by 20 – 50%

  • Increases the biogas production in anaerobic digesters by 20 – 50%

  • Reduces the volume of sludge to be transported drastically saving costs and carbon footprint.

  • Reduces the operating costs of sludge treatment by 40-50%

  • Low lifecycle cost.

  • Reduces odour significantly.

  • Reduces carbon footprint

  • It is compact and saves space

  • Cambi sludge storage saves 60-75% space compared to storage of sludge produced from conventional anaerobic digesters.  

  • In India transmission of diseases is a huge problem.  The high temperature used in the THP system gives guaranteed hygienisation.  Other Class A technologies give risk of Salmonella and thermo tolerant micro-organisms to be spread when farmers use the sludge in the production of vegetables and edible crops.

  • Use of Cambi cake makes it possible to reduce the amount of mineral fertilizer currently used. Mineral fertilizer increases the salinity in the soil, reduces carbon content and over time leads to reduction in yield, and even might take the soil out of production

  • Enables maximum recovery of phosphorous, either as struvite or in biosolids.


How do you plan to lead Cambi India in the coming years?


It is our responsibility that we leave a better and cleaner world for our kids. Today’s world is all about sustainability, circular economy, renewable energy, cleaner environment, safety, decarbonization, smaller footprint and costs. As discussed above, CambiTHP is one of the rare technologies that fits all. This puts us in an advantageous position compared to other Class A technologies.

Since the knowledge in sludge was not high in India, for the last 4 years we first took the initiative to educate the Indian audience about the need for sludge treatment to protect public health and environment through workshops, meetings and presentations. We are very glad to inform that we have received exceptionally good response. Today, utilities and regulatory bodies appreciates the importance of sludge treatment to produce hygienized and pathogen free sludge. Several tenders are rightfully requiring Class A standards for disposal sludge thereby creating a sludge market. This will protect the health of millions of Indian farmers and citizens. Now, several Class A technology companies have entered the market. Cambi is the world leader in Class A space with 81 references in cities of US, UK, Europe and Australia, China, S. Korea, Singapore and several other capital/large and medium cities worldwide. We process 50% of UK sludge and US is our fastest growing market. More and more municipalities around the world are choosing CambiTHP. Recently we signed up with city of San Francisco, Hongkong and several other cities.

CambiTHP is an advanced disruptive technology. It is a market differentiator. The benefits of Cambi technology is way more than other Class A technologies. We partner with those municipalities who seriously think about health and environment and to build a better world at a lower life cycle cost. For example, before 2014, Washington DC STP used to pay for sludge disposal. After installation of CambiTHP, they are selling the sludge as a fertilizer/soil amendment (www.bloomsoil.com).

Our strategy is to educate our clients with the benefits of installing Cambi not only from economic point of view but also from environment, circular economy and sustainability. Indian engineers are very bright and they can easily understand the benefits and can decide for themselves.  With Cambi, I will be happy to protect the health of the citizens, generate renewable energy and improve the soil conditions of the country. I believe it is just matter of time before a majority of municipalities adopt CambiTHP.

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