We egest 1.5 liters of urine and 150 grams of faeces every single day to our city’s sewers. Its ultimate fate is decided by the sewage disposal system of the area. But our waste along with other kinds usually ends up in a waste treatment plant where it is treated into clean water and solid waste called sludge. The sludge again is treated with various agents for removal of various pathogens and removal of odors. Common processes used are aerobic digestion, auto-thermal thermophilic aerobic digestion (ATAD), anaerobic digestion, composting, alkaline stabilization, thermal drying, including flash, rotary, fluid bed, paddle, disc, and infrared dryers, acid oxidation/disinfection, and heat treatment/acid digestion. What is left is a nutrient rich organic matter – Biosolids.
Biosoilds have been used for long as organic fertilizers by agriculturists and farmers. It has also been used to reclaim areas laid bare by mining and also to create fertile tracts for lawns and gardens.
But is Biosolids a green story?
Waste treatment and its resultant residues though come under various regulatory requirements. The US EPA (Environmental Protection Agency) says in its policy document, biosolids that meet treatment and pollutant content criteria ‘can be safely recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth.’
But this policy is not without its detractors. The common anti-stand against bisolids is that our sewage is not only made up of organic biodegradables but they also comprise of contaminants which are not broken down by the sewage treatment facilities. The list of contaminants include heavy metals (arsenic, cadmium, copper, etc. — some of which are also critical plant micronutrients) and toxic chemicals (e.g. widely used plasticizers, PDBEs, etc.) Then, there is the increasing consumption of compound chemicals in the form of medicines, preservatives and other household personal care products.
What happens when this chemical potpourri is poured back onto the land and finds its way into the food chain? There has been no systematic long term study of the effects of biosolids on our ecosystem in general and our health in particular. With significant rise in use (and abuse) of pharmaceuticals and personal care products, the very content of sludge has changed over the years. It hasn’t been tested because very few labs are kitted to do that kind of analysis, there is no standard methodology, and no benchmarks to say what’s safe. Incident health problems have been reported from different places but the direct relationship between sewage and health problems hasn’t been well documented so far. Some though call it the ‘toxic stew‘.
Sweden, Switzerland, France and Holland are among the countries that have either banned or introduced tougher standards on the use of biosolids as fertilizer. Today, they have started burning more of it as energy-from-waste.
Is recycling of biosolids a good idea? The categorical answer is ‘yes’. But then the question is how is to be done in a sustainable manner which does not raise public health concerns.
As of date, this issue remains controversial and fresh directives are expected.
Sources: Water Environment Federation