The De Nora blog: Water Made Easy

When Sludge Goes Bad: Marine Sewage Treatment ‘Class B’ Sludge Production

Sep 3, 2020 10:30:00 AM / by De Nora


So your marine sewage treatment plant (STP) is causing you to manage an unreasonable amount of wet sewage sludge. What can be done?

Many in the industry have looked to De Nora Water Technologies (DNWT), and the methods they use to produce Class B rated biosolids through the use of electrolytic treatment systems with optional offshore dewatering centrifuges.

The following are some of the critical factors and design elements in the electrolytic treatment system design that promotes this rated biosolids product.

To begin with, the rules and regulations for sludge/biosolids management start with the United States Environmental Protection Agency(EPA); U.S. Code of Federal Regulations 40 CFR Part 503.

This code/regulation sets forth the rules and guidelines for the use or disposal of domestic sewage sludge, including the allowable pollutant limits and management practices of waste sludges that are intended to be applied to the land(landfill) or surface disposal sites. Another critical parameter states that all sludges to be introduced into or on the land must be analyzed for the pathogen and alternative vector attraction reduction (VAR) requirements. The three notable biosolids groupings from the EPA are: 

Class A biosolids are those where pathogens have been reduced to virtually undetectable levels and achieve a high VAR.

Class A (E.Q.) exceptional quality biosolids exceed all Class A pathogen, reduction metals, and VAR requirements.

Class B biosolids are deemed treated but contain higher levels of detectable pathogens than those of Class A biosolids.

Both Class A and A(E.Q.) are allowed to be applied to land surfaces without restriction. This is not to say Class B biosolids are a direct danger to human health, but only that they are analytically measured constituents which are slightly elevated over those of Class A type; therefore, they must be managed according to the Part 503 requirements.

Because of the known powerful electrooxidation and sterilization processes of our marine electrolytic treatment systems, we decided to have the dewatered biosolids from our post-treatment step tested by a reputable EPA Partner laboratory at Tulane University, Louisiana, USA. 

Over an extended testing regiment, the lab performed the same land-based sludge analytical tests required to achieve Part 503 compliance. Since our systems operate in the marine environment, further assessment testing was performed to investigate the availability of the various toxins to the environment. The following tests must be passed to allow sludge disposal into the landfill scenario:

The toxic characteristic leaching procedure (TCLP): Determines if a waste is hazardous or a solid waste. 

The filter paint test: Determines if the waste residuals have any free water that could carry on pathogenic life.

The vector attraction requirements test (VAR), also known as SOUR test): Ensures a stabilized product is produced that won’t cause odors and/or attract pests(vectors) once placed in the environment.

In all required EPA tests performed on the dewatered biosolids from our electrolytic process, the residuals passed the requisite TCLP, filter paint, and VAR tests, which ensures that these residuals can be disposed of in a landfill. 

Even though Class B biosolids can be applied to land applications under specific Part 503 rules and management procedures, DWNT does not promote the use of their produced biosolids for surface land applications, farming or any form of domestic gardening or crop use, simply due to potential and/or perceived liabilities from region to region around the world.

To Store or Spin?

The benefits of managing these classified biosolids in the Marine and Offshore O&G industrial space are numerous. These include small installation space for a centrifuge unit as compared to alternate sludge holding tanks, safer handling by operators, little to no odor generation, no fear of producing a septic condition and safe disposal along with the other domestic garbage generated aboard the vessel or platform.

To Burn or Bury?

While incineration of solids wastes offshore is prevalent on some offshore installations (or vessels) as allowed per IMO MEPC. 244(66) Annex VI (Regulation 16), many offshore industrial operators list several reasons for forgoing incineration aboard these installations.

Many may find fuel costs too high, and operation of their incinerator difficult to maintain simply due to the actual daily waste loading being fed to the unit via the gathered refuse onboard. Others note that when the flue gas temperatures (per MEPC. 244(66)) can’t be adequately maintained, the incinerator is typically shut down. Sewage sludge alone has a relatively low calorific value of approx. 3,000 kJ/Kg, so without other fuel sources entering the incinerator, it can easily fall out of regulatory compliance or cost too much to continue firing without a consistent waste load. In other words, it is merely much easier for offshore operators to send their refuse ashore for landfilling or further treatment.

Misuse of terms

In oil extraction and production, “sludge” often refers to a thick, viscous emulsion containing oil, water, sediment, and residue. It forms because of the incompatibility of certain native crude oils and strong inorganic acids used in well treatment. In fact, there isn’t one reference for sewage sludge found in the MAPROL Annex I, Regulations for the Prevention of Pollution by Oil. So why is there so much confusion on this matter?

Over time, the use of various terms such as sludge, activated sludge, biosolids, wet waste, desludging and solids waste discharge are tossed around at will. This has undoubtedly led to a level of confusion and uncertainty with many crew and operators aboard vessels and Offshore O&G platforms. This could be due in part to the fact that many of these same terms are used onshore, primarily at municipal waste treatment plants, but have slightly different meanings.

Indeed, we must give the benefit of the doubt to the manufacturers of biological-based marine STPs as, over the years, they have used these terms interchangeably; meaning that these terms are similar to the land-based municipal wastewater treatment terms, but do not necessarily define the actual biological action that is happening.

While sludge is a wet waste in the marine world, it is more accurately defined as the waste oils collected from the onboard separators. Note that sludge (wet waste) tanks must be in accordance with MARPOL guidelines, and fluid transfers in and out of that tank must be recorded in the oil record book where the MARPOL Convention is applicable. At present, this is not a process that should be duplicated for the sewage sludges generated from onboard STPs.

Sewage sludge activated sludge is the solids separated or extracted before or during the treatment process of domestic wastewater. Depending on the effectiveness of the treatment process, the removed sewage sludge produced may or may not be considered stabilized and, therefore, can remain a potential human health hazard depending on the vector attraction reduction (VAR) and pathogenic levels. 

Biosolids, on the other hand, is a term widely used to refer to those materials (sewage sludge) that have undergone additional conditioning to meet specific EPA pollutant and pathogen requirements for land application and surface disposal. 

We are proud to say that Class B biosolids produced by De Nora electrolytic treatment systems are the only known classified biosolids generated by an International Maritime Organization (IMO) compliant offshore marine sewage treatment plant.

Does Your Water Treatment System Have What it Takes?

Be sure you fully understand the capabilities of your water treatment system before it’s too late. Download our FREE Guide to Sewage Treatment for Offshore Oil & Gas. 

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Tags: water treatment, De Nora, Sewage treatment

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