Dental Tribune India

Casting a green future: Recycling the gypsum dental products – Sakshi Sharma

By Dental Tribune South Asia
July 03, 2020

The prospects of green, eco-friendly, sustainable dental practices are the need of the hour which necessitates the reuse and recycling of dental waste e.g. gypsum products. Gypsum waste produced in dental settings includes the dental stone, plaster of Paris, gypsum investment material and dental plaster. This article describes various methods used for the recycling of gypsum products.

Introduction:

Do you know what the Great Pyramids of Egypt and denture patients’ cast models have in common? They’re both made out of gypsum products! Today, Gypsum plaster and stone make up a huge chunk of dental waste generated every day across dental establishments.

Its Effects?

When landfilled, gypsum waste anaerobically breaks down to produce characteristically rotten-egg smelling, highly toxic hydrogen sulphide gas. The most sensitive targets of this gas are the respiratory tract and nervous system, posing public health at risk by and large. Some of the notable symptoms include irritation in eyes, nose, or throat besides causing headaches, dyspnoea and over a long period of time, it may result into poor attention span, weak memory, compromised motor functions, lethargy and vertigo.

If incinerated, gypsum releases sulphur dioxide, a toxic gas that pollutes the land, air and water alike.

Fig.1: Great Pyramids of Giza, Egypt. Source: @svstrelkov, Freepik
Fig.2: Dental Casts, Source: Will Devine, Whipmix

The gypsum products pose an enormous environmental impact even before the packet is opened. The amount of destruction and waste produced during mining, manufacturing and packaging is insurmountable.

Amount of waste generated?

It can be estimated that each dental college produces between 100 to 500 kg of gypsum waste depending on the patient footfall per year. We have around 345 dental colleges in India. This means 173 tonnes of gypsum waste is generated alone by colleges. Adding dental practitioners around the country would almost double the figures, assuming there are 3 lakh practising dentists in India currently. This is just the waste produced by dental-related practices. Imagine the staggering numbers when combined with medical/ orthopaedic waste, construction material and industrial waste.

What can be done?

The first step would be to segregate gypsum waste from other wastes so it can either be recycled and reused or disposed of properly at a landfill. It can be disposed of responsibly in accordance with the Environment Permitting (England and Wales) Regulations 2010, which requires it to go into a separate cell meant for high sulphate waste.

Fig.3: Hydrogen Sulphide Fumes, Source: @irinacliva, Freepik
Fig.4: Open Pit Mining for Gypsum, Source: Tom Fisk, Pexels
Fig.5.: Irresponsible Disposal, Source: user7264515, Freepik

Recycling is the most indispensable method to deal with gypsum waste as 99% of recycled gypsum is as good as virgin gypsum.[1] Segregated waste should be thoroughly cleaned to remove remnants like alginate, disinfected and sterilized using 2% glutaraldehyde. [2] Recycling can be carried out in two ways, chemical method and physical method. The chemical method collaboratively developed by National Chemical Laboratory, Pune
and Institute of Chemical Technology, Jalna. The method involves the use of 20% ammonium bicarbonate solution. The reaction disintegrates the gypsum and produces a sludge which contains non-toxic products, namely, calcium bicarbonate and ammonium sulphate within 24 to 36 hours at room temperature.

Ammonium sulphate can be used in textile, pharmaceutical and wood industries besides the manufacture of nitrogen-based fertilizers and fire-extinguishing powder. Calcium bicarbonate, on the other hand, finds use in metallurgical operations for reducing ores of less reactive metals than calcium such as in the manufacturing of steel. [3]

The physical method involves what’s called Closed Loop Recycling. Gypsum is eternally and infinitely recyclable. For instance, Plaster of Paris is manufactured by heating raw gypsum or calcium sulphate dihydrate to remove the chemically combined water. Once mixed with water and hardened, set Plaster of Paris is chemically the same entity as gypsum. This too can be cleaned and disinfected, broken or ground into small chunks and heated at an excess of 325 °F or 120 to 170 °C, thus producing Plaster of Paris. A similar process can be carried out with dental stones, dental plaster and to an extent, even gypsum bonded investment
material. The recycling process does not modify the chemical composition, physical or mechanical properties.[4]

Fig.6: Recycling bucket for dental casts, Source: https://www.citronhygiene.co.uk/
Fig.7: Recycling, Source: @macrovector, Freepik

Reuse of gypsum waste without the need to modify it chemically or physically can be less tedious. It can be done in a variety of ways:

1. A Great Source of Valuable Plant Nutrients, for Remediation and as a Compost Additive:
As a fertilizer, provides a moderately soluble source of calcium and sulphur, increased crop yield in many plants such as peanuts, watermelon, tomatoes and apples, to correct soil acidity, prevents plant disease.
2. Reduce Toxins in Agricultural Runoff: Studies show a 55% reduction of soluble reactive phosphorus in runoff water in gypsum-treated plots of land when compared with untreated plots. These findings show a significant potential to improve water quality in agricultural areas.
3. Additive for Animal Bedding: Organic materials, such as sawdust, shavings, straw support conditions which are ideal for pathogenic growth. Adding gypsum additives to dairy bedding keeps bedding dryer thereby keeping farm animals like cows, sheep, horses, etc safe.
4. Bulk Waste Solidifiers & Stabilizers: Liquid waste is often converted into solids to minimize the release of harmful chemicals from industrial waste sites, gypsum powder is an ideal choice to convert semi-solid slurries for faster waste cleanup and remediation.
5. Fillers & Diluents: Recycled gypsum is used in sealants and speciality types of cement. Due to its inert properties, it is an ideal diluent for a variety of products including Plastics, Adhesives, Paint, Wood Filler, Textiles, Modelling Clays.etc.
6. Water Clarification: Cloudy or muddy water in ponds is not aesthetically pleasing and can be potentially dangerous, which can be detrimental to aquatic life. The addition of gypsum helps to clarify the water & will not affect the pH of the pond or lake.
7. Lawn & Garden: The key to lush, green grass and blooming gardens is proper soil conditioning. Gypsum loosens clay soils, naturally aerates and improves water penetration. This encourages the production of early roots. Recycled gypsum soil amendments are cost-effective and safe for pets, people and plants.

Fig.8: Farming with Gypsum waste, Source: @ilixe48, Freepik
Fig.9: Bedding with Gypsum waste, Source: @jcomp, Freepik
Fig.10: Industrial applications, Source: @macrovector, Freepik
Fig.11: Green Lawn, Source: @4045, Freepik

Inspiring the change:
At the front lines of this new and sustainable way to keep gypsum waste at bare minimum are two organizations we must take inspiration from. Bharati Vidyapeeth University’s Gypsum recycling unit was
established in the affiliated dental college in collaboration with their affiliated engineering college in Pune. The unit has 50 kg of plaster of Paris and 40 kg of dental stone from waste gypsum products in the session 2019-2020. The Pune Municipal Corporation is tackling the challenge of water body pollution due to immersion of Ganesha idols made of gypsum using the chemical recycling method developed by National Chemical Laboratory, Pune.

Fig.12: Ganpati Visarjan, Source: Sanjib Samanta, Pexels

Fig.12: Ganpati Visarjan, Source: Sanjib Samanta, Pexels

Concluding Thoughts:

There is a lack of interest, knowledge and participation from the government and waste management system which makes us all the more responsible for the proper use and reuse of gypsum products. The advantages of reusing and recycling or even carefully disposing of gypsum are very obvious. The natural gypsum reservoirs can be conserved and carbon emissions are also reduced. Freed up landfill space can be used for other materials that just cannot be recycled or reused. Following a few simple, mindful steps can help decrease the
combined impact that dentistry has on our planet. If you are still unsure how you can contribute towards a greater good- start small. Begin by segregating waste and keeping a log. If all else fails, a waste management consultant can always be hired to make your dental setup greener and sustainable.

References:

1. Fairweather RJ, Barlaz MA. Hydrogen sulfide production during decomposition of landfill inputs. J Environ Eng 1998;124:353-61.
2. Ivanovski S, Savage NW, Brockhurst PJ, Bird PS. Disinfection of dental stone casts: antimicrobial effects and physical property alterations. Dent Mater. 1995;11(1):19‐23. doi:10.1016/0109-5641(95)80004-2
3. Rapid and greener method for utilization of Plaster of Paris (POP) waste generated from biomedical samples GR Navale, KN Gohil, KR Puppala, SS Shinde, S Umbarkar, MS Dharne International Journal of Environmental Science and Technology 16 (5), 2475-2480
4. Shiyo, S., Nagels, J. & Shangali, H.G., 2020, ‘Recycling of plaster of Paris’, African Journal of Disability 9(0), a503. https://doi.org/10.4102/ajod.v9i0.503

Author:

Dr Sakshi Sharma is a BDS Intern at Seema Dental College and Hospital, Rishikesh. A University Gold Medallist, she has several awards and scholarships to her name. Notably, she was named Guident Student of the Year in 2017 and won the Root Canal Foundation's 8th Annual Young Achievers Scholarship in February 2020. As a student, she has presented a number of topics at various national and international conferences and holds a keen interest in innovative and evidence-based dentistry.

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