“What are biosolids?
In the past few decades, the primary purpose of wastewater treatment utilities has been to treat sewage water and release it into the environment as effluent without causing degradation or harm. The often unmentioned and unpopular by-product of this process, sewage sludge, is going through a challenging yet promising epiphany that alters the purpose of such facilities. This brown, viscous semi-solid material was often treated as a public nuisance and a waste item. However, once treated with certain technologies, it transforms into what is now dubbed “biosolids,” an alternative soil product or fertiliser that, depending on its level of treatment, can be used to feed nutrients back into agricultural lands and other depleted environments. The term “biosolids” is now widely used in the global wastewater industry to refer to treated sludge.
Legislation in the United States allows biosolids to be managed in a variety of ways from landfilling to beneficial reuse, with land application or recycling encouraged by the US Environmental Protection Agency. Such biosolids management methods, along with compliance limits for biosolids, and the different classes of the material are laid out by the same agency in a comprehensive policy called the US EPA Part 503 Biosolids Rule.
What is the US EPA Part 503 Biosolids Rule?
Promulgated in 1993, the United States Environmental Protection Agency (US EPA) Part 503 Biosolids Rule, also called the 40 Code of Federal Regulations or CFR Part 503, establishes treatment and management standards for sewage sludge and septage. The comprehensive policy affects all US-based sludge treatment plants. It is also used by the Water Environment Federation (WEF) and closely mirrored or studied by other countries looking to modernise sewage sludge treatment, such as India.
The rule classifies biosolids into two categories: Class B and the higher-quality Class A. Though there are only two major classes in the policy, each has subclasses or “options” that are part of a more complicated matrix. A simplified version of this is found in the table below. Of note is a type of Class A biosolids rated “exceptional quality” or EQ. These represent the cleanest and safest form of biosolids in the country and can be used on residential lawns and gardens.
The main difference between Class A and Class B biosolids is that the former does not register any pathogens while complying with the most stringent limits for pollutants in the US EPA Part 503 Biosolids Rule. Class A biosolids are often produced through anaerobic digestion and additional treatment, such as heating, pasteurisation, composting, or newer complementary technologies. Biosolids that achieve Class A status may be used as fertiliser, soil amendment, or soil conditioner for public contact sites such as agricultural land.
There are three applications of Class A and Class B biosolids identified in the rule that reflect how most modern cities already conduct biosolids handling. These are the following:
- Land Application, identified as the use of biosolids on non-public contact sites such as agricultural land, forests, and reclamation sites (e.g., mined areas) or public contact sites such as parks, golf courses, roadsides, and plant nurseries. All biosolids that achieve Class A or Class B status can be land applied in the United States, as long as they meet the specific requirements for each subclass and area of application. Use in lawns and home gardens is also allowed but only for particular subtypes of Class A biosolids.
- Surface disposal, which involves the placement of biosolids, Class A or Class B, in monofils, dedicated disposal sites, waste piles, or surface impoundments and lagoons. There are also “dedicated, beneficial use sites” – agricultural lands that must secure that precautions are in place to protect the environment and public health in case of the adverse effects of pollutants on crops or animals. All these areas must abide by specific requirements and pollutant limits based on the proximity of the biosolids to the site’s boundary. However, disposal in a municipal landfill is not considered surface disposal and falls under a separate regulation.
- Incineration of biosolids, which entails heating the biosolids at very high temperatures using an enclosed device. Incineration requires a furnace, air pollution control devices (APCDs) and auxiliary fuel that often contains a mix of other organic wastes or municipal solid waste.
The table below summarizes the requirements for achieving Class A or Class B biosolids according to the US EPA Biosolids Rule. There are various types under each class that determine how they are handled.
Not only safe but beneficial:
Land application or recycling of biosolids
Of the three alternatives mentioned above, there is a growing inclination for agricultural land application in countries with sufficient land bank and permitting regulations. This favour for land recycling, which accounted for over 60% of biosolids use in 2018, is owed partly to its economic advantages. It turns what was previously thought of as waste into a resource and may lessen operational expenses substantially. A study in 2019 about the costs of sludge treatment in the EU, for example, noted that the cost ranges per metric ton of raw sludge handled were lowest for land application.
The support for biosolids land application is also strongly connected to the regulatory actions phasing out landfilling and ocean dumping of sludge or biosolids globally. Even in the US, where landfilling is allowed, only 22% of biosolids from publicly owned treatment works in 2019 ended up in the landfill versus 51% land applied. This speaks to the disadvantages of landfilling: the intensive site evaluation, planning, operation, and closure; the potential for groundwater contamination from leachate; odour generation; and loss of resources. It is also important to note that though land recycling is the most favourable option, incineration remains practical and widespread, especially in countries where biosolids land application is not permitted by law. In the US, around 16% of US public treatment plant biosolids were incinerated in 2019.
Benefits of land application of biosolids
Though it is expected to grow to a value of 1.9 USD billion by 2025, the development of the global biosolids market is still fraught with the challenge of educating whole populations about the benefits of beneficial reuse. Negative public perception remains significant for governments considering biosolids land application. Local assurance schemes and studies are a good tool to guarantee the public’s confidence in this area. The United Kingdom’s Biosolids Assurance Scheme is one such example, set up by the UK’s primary water and sewerage companies to regularly audit and provide certifications for biosolids producers, transporters, and users. A 2015 research effort prepared for the Canadian Water Network is another exemplary component of a country’s campaign for land application. Two local studies were performed to assure the Canadian public about the positive impact of biosolids on crops and certain invertebrates and the odds of heavy metals leaching into soil and food.
A growing preference for Class A biosolids
Class A biosolids and its subtype Class A – Exceptional Quality (EQ) currently account for half of the global biosolids market share. This share is expected to grow as it presents minimum risk compared to its less stringent counterpart, Class B biosolids, which perhaps represent that which leads to public scepticism about processed sewage sludge in general.
Though the mixture is allowed to have detectable pathogens and is frequently more odorous, biosolids that achieve Class B status have been proven safe and even advantageous for land application in cases where restrictions were complied with. That said, some reports like one done in 2002 by the University of Georgia still help fuel the inclination for biosolids that follow stricter standards than Class B. The particular study found that a quarter of over 50 residents in eight US states living near sites for Class B biosolids had Staphylococcus aureus infections due to the wind blowing particles from the site to their homes. It caused burning sensations in the eyes, throat, and lungs, as well as skin rashes. Such findings urge greater vigilance in the use of Class B biosolids and serve as an endorsement for upgrading to achieve better quality cake.
The production of Class A biosolids is also becoming more easily attainable with the emergence of treatment technologies that work in conjunction with anaerobic digestion. Though the Biosolids Rule identifies some technologies that achieve Class B and Class A standards, the list in the document could greatly benefit from an update, given that a host of new methods have gained prominence in the past couple of decades. Thermal hydrolysis, for example, is a process technology that broke through in the United States in 2011 with a contract for DC Water, one of the world’s leading utilities. The thermal hydrolysis technology supplier, Cambi, now has several references in the country producing Class A biosolids via advanced anaerobic digestion. DC Water even went into the retail and wholesale marketing of their exceptional quality biosolids under the product name Bloom.
As countries worldwide develop economically and environmentally, similar requirements and limitations for high-quality biosolids may develop. Though it is difficult to say whether there will ever be a global standard for the classifications of biosolids, it is possible that the conditions as the EPA 503 Rule currently prescribes them will be influential. We can also expect that the biosolids management methods in developing countries will grow to be more varied like in Europe and the United States – adapting land application, surface disposal, incineration, and similar methods as their cities grow and large treatment plants flourish. Beyond adherence to pollutant limits and precautions, Class A biosolids treatment technologies will also help usher in a future where biosolids are a sought-after, fertile resource and no longer just a risk-laden by-product.”
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