On 1 January 2010, the International Maritime Organization's (IMO) MEPC.159(55) effluent standards and performance tests for sewage treatment plants installed on board went into effect. Marine sanitation device (MSD) equipment manufacturers offer a variety of treatment schemes they claim effectively treat wastes for the offshore exploration and production markets. But to properly determine the efficacy of an MSD, closer evaluation of the logistics of its specific solids/sludge handling system should be made against the following criteria:
Where does the sludge/solids waste come from?
- Where does the sludge/solids waste come from?
- Where do the solids go if they can't be discharged to the sea?
- How are the sludge/solids handled by the particular MSD system?
- Are the sludge/solids safe to handle?
Sludge results from wastewater treatment where chemical or biological treatment processes are used to convert dissolved or colloidal substances into suspended solids of large dimension. Solids include physical mass solids (biomass) of wastes in the sewage stream combined with degrees of grit and fibrous materials (mainly cellulose from toilet tissue). These residual byproducts, sludge and solid waste, must be extracted and handled properly to prevent sickness and possible infection to persons operating or maintaining the MSD equipment.
Where do the solids go if they can't be discharged to the sea?
Solids remaining in the waste stream of an MSD system that can't be broken down must be removed physically or pumped to an alternate treatment or handling system.
Managing the sludge and solid waste through an MSD system can include techniques ranging from simple pump out to an alternate holding tank to extensive pre-screening and solids separation of the raw sewage inlet stream.
How are the sludge/solids managed by a particular MSD system?
There are various approaches to managing the sludge and solids from the waste streams. Certain MSD equipment requires removal of untreated raw solids prior to the treatment unit while others allow the biomasses to build over time, requiring removal of the wastes as a maintenance step.
A user should fully examine the solids handing capability of any MSD unit, as some units may require extensive solids handling systems — a requirement that may not be clearly indicated.
In truth, many physical-chemical, moving bed bioreactor (MBBR) and membrane bioreactor (MBR) systems will not perform as advertised if the solids loading into the system's processing sections are not reduced. Unfortunately, this requires the removal of specific solid wastes prior to treatment within the MSD. This presents a valid health and safety concern to operators and those who come into contact with the sewage treatment unit.
Some traditional biological and physical-chemical MSD systems have increased in size due to additional biological/membrane tanks or certain chemical addition systems required to meet effluent discharge standards. As with any aerobic degradation process, there is a certain amount of sludge produced by the biological process. These solids normally settle in the bottom of the biological activity tanks and require scheduled pump out and possible physical removal of the remaining sludge by hand. An overload of influent sewage sludge or solids (biomass buildup) decreases the unit's ability to effectively destroy the fecal matter, resulting in increased solids buildup in the tanks.
MBBRs and fixed film-based treatment systems can also be negatively affected by biomass buildup to the point where the units lose their treatment ability and become clogged with solids. Often, continual washout of screens and filters is required — and washout waters are not considered "treated" and must undergo additional treatment or suitable disposal.
The more advanced MBR systems provide an absolute barrier against passage of total suspended solids (TSS) and finer fibrous constituents to the discharge of the unit, but at a cost. These systems usually require pre-mix or combination volume tanks to allow for greater solids buildup in the bottoms of the biological tanks, or they utilize unsanitary pre-screening and solids removal from the untreated influent waste stream. During the normal operation of the membrane system, routine maintenance time is required to de-sludge, or pump out the tank bottoms to eliminate the solids/sludge buildup. This pump out routine can easily take hours to complete and is often coupled with the membrane chemical cleaning regimen required by the supplier. During this down time, the customer is left without essential waste treatment, and some MBR suppliers recommend an additional MBR system as a backup during this long maintenance downtime — a significant added expense. Finally, the de-sludging process requires the handling and appropriate discharge of the voluminous sludge and cleaning agents.
Are the sludge/solids safe to handle?
Before deciding on a particular MSD technology, the entire treatment process must be evaluated with regard to the "realistic implementation" of the system aboard a vessel or oil and gas production platform. The customer should perform an exhaustive review of the sewage treatment system's solids/sludge handling techniques prior to installation to avoid potential health and safety issues.
Health and safety standards should be reviewed to ensure that workers associated with offshore-based MSD systems understand the proper solids waste handling duties and logistics of disposing of wastes. Prior to 1 January 2010, the effluent discharge criteria of the IMO/USCG were less stringent, and solids handling was not really required due to MSD unit treatment techniques. Now, many operators are faced with specific handling and disposal logistics of the sludge/solids which can become overwhelming.
Managing a waste sludge liquid stream of one to two percent solids is quite different than managing captured, dewatered solids masses in the 10 to 20 percent solids range. Depending on a particular vessel/platform's on-board waste handling procedures and health and safety logistics, utilizing certain types of sewage treatment units could result in additional operational attention, not to mention unsanitary and unhealthy working conditions. The USEPA Part 503 Biosolids Rule of 1993 divides sludge into two classes based on pathogen content. Class A biosolids are those in which pathogens are below detection levels. Class B biosolids are those which have detectable, but acceptable levels of pathogens. These pathogens may rapidly die off when applied to soils under Part 503 Rule requirements, essentially becoming pathogen-free within a short period following application. Both classes of biosolids may be land-filled when applicable.
Individual waste handling operating procedures and environmental compliance regulations will vary in every region of the world, so it is important to understand the impact of the various solids/sludge handling methods available in each local jurisdiction. Disposal and/or transportation permits may also be required in certain regions of the world.
Electrolytic MSD proven effective
Severn Trent De Nora's OMNIPURE Series 55™ MSD systems are the only proven electrolytic marine sanitation devices that offer an environmentally safe and sanitary method of managing the produced solids from the treatment process. Since OMNIPURE's unique electrolytic treatment process effectively disinfects the waste stream via aggressive oxidation within the unit's electrolytic cells, it does not require a biological colony of organisms to destroy the fecal coliform bacteria and does not require removal of normal sewage waste solids prior to treatment. This technology allows for the TSS/fibrous material in the process stream to be subjected to the powerful oxidant, sodium hypochlorite, thereby sanitizing the TSS material. These TSS/fibrous solids in the process stream are then electrolyzed in a secondary process of electrocoagulation, which causes agglomeration of the fibrous matter, which then settles to the bottom of the unit's concentration chambers. The agglomerated solids are then removed automatically from the process while the system is in operation without any disruption to the treatment ability of the unit.
With two available solids handling systems as options, Severn Trent De Nora's approach to managing the solids produced by the treatment system is focused on operator health and safety as well as the system's ease of sludge/solids disposal. With solids handling occurring at the unit, the handling of raw influent sewage wastes via pre-screening, filtration or separation prior to an MSD's treatment system is not required.
The dewatered solids produced by the OMNIPURE system have been carefully evaluated, tested and proven safe by an independent third party analysis (Dr. R.S. Reimers of Tulane University, Department of Environmental Health Sciences, Ph.D., Q.E.P, FAIC; to be published in the near future), to meet the classification of "Class B" sludge. Unlike many other MSDs, the OMNIPURE's dewatered solids can be land-filled or disposed of by traditional waste management methods without fear of environmental impact, penalties or a risk to life and health.
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