Solar drying sludge folder: article 1
There is currently a waste management problem with the quantity of human waste sludge generated in the world today. Indeed, we have never produced so much sewage sludge and we have many difficulties in eliminating or valorizing them without degrading the environment, and without spending a lot of energy to resolve these issues. With the increasing world population growth this problem is only worsening. Active solar drying of sewage sludge seems to be one of the most innovative ways to find a solution to this problem. But first and foremost, why does water treatment generate so much sewage sludge and how do the authorities currently manage, eliminate or exploit this significant sludge intake each year?
History of wastewater treatment
In the 1960s and 1970s, the growth of the world's population in the West, and the previous habit of directly discharging industrial, commercial and municipal sewage into the waterways had severely degraded rivers around cities. The direct disposal of sewage sludge into these waterways created serious human health problems. So, we developed techniques to treat wastewater from sewers with water treatment plants, as was the case in Montreal in the 1970s. These changes in perspective and different standards were also applied to the industry during this period.
Even today, in Africa, Asia and Latin America, many cities do not treat their wastewater. The fecal coliforms and the various pollutants are then found in the water that people consume, causing many problems for human and animal health. (United Nations World Water Development Report, 2017)
Hundreds of millions of tons of sludge produced each year worldwide
Sewage sludge is the main waste byproduct of wastewater treatment. In Quebec in 2012, nearly 1 million tons of sludge was produced (not counting industrial and commercial sludge). In the United States, there was more than 6.5 million dry tons a year produced in 2006. Typically, a water treatment plant treats wastewater from local businesses as well as residential wastewater within a municipality. They can accumulate different elements (oil, bacteria, chemicals, hormones) that can complicate the treatment of the resulting sludge. Large industries: pulp and paper, mining and oil, are also required to do their own treatment of their wastewater and dispose of their sewage sludge according to government standards.
Therefore, we produce a high volume of sludge, but what are the different treatments they have undergone before leaving the treatment plants? Water treatment can be very complex in order to produce a valuable sludge. The sludge must be transformed through a series of treatment steps described below to be easily valorized.
The main stages of wastewater treatment:
- Screening and sieving:
Screens retain the large debris that is contained in the sewage water: wood and plastic, leaves, fabrics, etc. Then sieving further with increasingly reduced screen sizes can complete this first step.
- Sand and fat removal:
The flow is slowed down to allow the decantation of sand and small gravel at the bottom of the first basin. Then the water goes into a second pool where the fats are pushed to the surface thanks to the action of microbubbles. The fats are then scraped off to recover them. We thus harvest the first part of the primary sludge at the bottom and top of these basins.
- Biological treatment (activated sludge) or physicochemical treatment (flocculating settlers):
This is where we get the most sludge. In the case of biological treatment, oxygen is artificially added, which allows bacteria to degrade the suspended organic matter more rapidly, the dead bacteria and the decomposed material are then deposited in sludge at the bottom of the pond. With respect to decanter-flocculation system, a coagulating agent is added which will make it possible to artificially coagulate the organic matter in suspension. Floccules will be found on the surface or at the bottom of the tank, where they will form sludge.
Filtration using sand beds, UV radiation, electrolysis, chemical treatment and activated charcoal: these different processes make it possible to remove small particles from water, like bacteria, viruses or pollutants such as nitrogen and phosphorus. However, sand filtration is the most common tertiary treatment.
It's about treating micropollutants like traces of hormones, pesticides, or cosmetics using different methods. Quaternary treatment is not systematically applied to all treatment plants and research continues to improve this step: for example, a fungus-based treatment can be used to remove a large proportion of these micropollutants, or to compost sludge for the same reasons.
How to dispose of or reclaim used sludge
Once the sludge is produced, there are three main markets to either eliminate or reclaim this sludge:
- Land or forest fertilizer, mainly sourced from municipal sewage sludge
In Quebec, land application is allowed on crops where neither fruit nor vegetables are grown. However, they may be used in this case if the biosolids are certified by the BNQ(Bureau of Standardization of Quebec) proving untraceable contamination. Spreading was controversial and banned in several countries because of potential problems with human health (heavy metal and bacterial contamination problems). Now the spreading of these biosolids must follow a highly managed and this solution is the most carbon-neutral solution. The risk to human health is almost zero and this is a solution that many governments prefer today.
In addition, the tertiary and quaternary treatments of sludge more effectively eliminate pathogens and some problematic residues. Sludge granular drying, composting and fungus mycelium application may also be other treatment techniques that can facilitate its subsequent application. Concerning the landfill, it can generate human health and environmental problems such as significant greenhouse gas emissions. Quebec's new waste management policy therefore bans the burial of organic materials from 2020 onwards.
Incineration has the advantage of producing ashes that can be used as fertilizer, reducing the volume of waste sludge and limit human health problems. However, it is necessary to control the production of NOx gases, which are very harmful greenhouse gases.
The Drying Challenge
But for these three solutions, the main problem is the high-water content of the sludge. Drying is therefore a major issue for sludge valorization. To spread, bury or incinerate, you must first transport the sludge and high moisture increases transportation costs. Moreover, drying will facilitate their treatment and handling for spreading or for landfilling. For incineration, a drier sludge improves boiler efficiency and reduces cost. Sludge drying is therefore a crucial step, however current drying processes consume a lot of fuel and real estate.
This is why solar drying sludge represents a solution for the future. Passive solar drying solutions have already been developed with greenhouses and drying basins, although no solar concentrating active solution for drying has been designed and tested until now. (In the coming months Rackam will feature an article about concentrated solar drying solution and comparative cost with other processes).
Sludge Drying by Solar Concentration
The first active solar drying solution for sewage sludge has been successfully tested by Rackam and Heat2hydro for the city of Surprise in Arizona this year with promising results. That's why Rackam believes in this solution that has potential for applications here in Quebec, but also around the world.
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