Connecting ship’s food waste to the sewage or greywater system and thus the sewage treatment plant of the vessel causes non-compliances to the MARPOL Convention and national biosecurity laws.

Recognising the pollution impact and the biosecurity risk of ship’s food waste, the IMO’s MARPOL Convention regulates its disposal, recording, and reporting requirements. MARPOL Annex V stipulates three legal food waste disposal routes. Ship’s sewage or greywater system is not one of them.

Connecting food waste to the sewage system of a vessel also violates Annex IV (sewage) of the MARPOL Convention. When waste streams of different MARPOL Annexes mix, the most stringent requirements shall apply. However, the discharges of the affected sewage and grey water systems are not labelled or managed as food waste. Food waste is made to disappear.

Food waste from international ships also carries biosecurity risks such as plant pests and diseases such as swine fever, rabies, and avian flu. Many countries have strict biosecurity regulations to protect their agricultural interests (table below). These national laws often prohibit the discharge of food waste into the territorial waters. They also stipulate strict procedures for storing, transporting and disposing such food waste when landed ashore. However, these national laws are violated when food waste is discharged into national waters and municipal sewers in disguise as effluent, grey water, sewage, or sewage sludge. Such non-compliant piping design is a non-compliance which can last the life of a ship. Wrongly approved by classification societies, it has been spreading across the shipping industry over the past decades.

The consequence can be far-reaching. The practices have violated the international and national laws, contributed to the poor performance status of sewage treatment plants onboard, undermined the credibility of the approval regimes, forced ship masters to authorise false entries in the Garbage Record Books, and damaged the reputation of the maritime industry especially when the aviation industry has managed to stay in compliance with the national biosecurity laws.

So far, no relevant authorities have aired their opinions or taken corrective actions. The shipping industry continues to be misguided by certain class rules. It may take an awful long time for the regulations to be effectively implemented. But there are signs of constructive change initiated by the industry.

The Cruise Lines International Association (CLIA) released an informative report in 2021 on the cruise industry’s commitment to responsible tourism practices and environmental technologies. It includes a case study of a liquid derivative generated from the food waste processing equipment. It stated that ‘CLIA members treat this liquid waste as food waste as governed by MARPOL Annex V Regulations 4 and 6 which set forth the maximum food particle size, discharge rates, and distance from the nearest land or ice-shelf when discharged’ [1]. This is a concrete step towards future-proofing compliant system design, installation, and operation.

Continue reading

Having a sewage mixing tank with built-in aeration upstream of a sewage treatment plant onboard ships is not just a good practice, but a strategic move that ensures the future-proofing of new ships. The benefits of this setup are not only numerous but also significant, as we’ll delve into.

WHAT IS AN AERATED SEWAGE MIXING TANK?

A mixing tank, positioned upstream of a sewage treatment plant, is a crucial component in the sewage treatment process. It balances different waste streams, buffers peak flows, minimizes health and safety risks, and ensures a stable treatment process. It also helps the sewage treatment plant meet its certified hydraulic capacity and comply with existing and future rules.

A mixing tank is distinct from a designated sewage holding tank, which is approved by the authority and stipulated in a ship’s International Sewage Pollution Prevention Certificate (ISPPC). The capacity of a designated sewage holding tank is determined by the Administration, taking into account the operation of the ship, the number of persons on board, and other relevant factors. Typically, a sewage holding tank provides black water a hydraulic retention time (HRT) of 8-10 days on cargo ships, and 1~3 days on large passenger ships. In comparison, a mixing tank is based on industry know-how (not by approval) and takes into account the sewage treatment plant hydraulic capacity and the daily flows of all waste streams (not just black water) to be treated. Typically, it provides a hydraulic retention time (HRT) of about 1 day on cargo ships or less when the sewage treatment plant has a higher peak flow factor or when the flows are less fluctuated.

THE SEWAGE MIXING TANK: A CATCH-ALL FOR BALANCING AND BUFFERING

One of the most crucial functions of an aerated mixing tank is managing sewage treatment plant influent flows (Qi). By capturing all sewage treatment plant influents in one place, the tank ensures that all flows and characteristics are accounted for, a fundamental aspect of effective sewage treatment.

It should be noted that gravity or vacuum (e.g. integrated ejectors) influent into a sewage treatment plant can be problematic because it is impractical to measure its flow or to take a representative sample. Likewise, on many ships, grey water is connected to the last stage of a sewage treatment plant, causing non-conformities and poor sewage treatment plant performances.

Therefore, it is a good practice to capture all waste streams in an adequately sized mixing tank to buffer the peak flow conditions and balance the different characteristics.

EXCURSION: HAMANN AG’S APPROACH TO SIZING SEWAGE MIXING TANKS

The recommended size of the sewage mixing tank cannot be calculated with a general rule. It depends on several factors, which have to be considered and weighted individually for each project. HAMANN AG takes into account the following aspects:

• Expected total volume of black water and grey water per day
• The recommended mixing ratio between black water and grey water of about 1:5
• Peak volumes, e.g. in the mornings and evenings
• Run time of the sewage treatment plant feed pump should be about 1 hour before it is stopped again to avoid excessive start/stop operation

AERATION IS ESSENTIAL

A sewage mixing tank needs to be mixed to provide a homogenous influent to the sewage treatment plant. Mixing the tank using aeration is beneficial, if not essential.

Anaerobic conditions in sewage systems can lead to the production of toxic and flammable gases. This is an unacceptable hazard within the confined boundaries of a ship. Various safety features can be incorporated into the design and operation of a sewage system. Having barriers between the sewage gases such as the water traps, ventilation of the tanks, etc., should be considered as secondary measures of protection: the prime safety feature is to prevent the production of hazardous gasses within the system in the first place. The design of a tank holding sewage may include features for maintaining an adequate oxygen level in the liquid so as to eliminate anaerobic conditions. The IMO Maritime Safety Committee issued “Guidelines for the operation, inspection and maintenance of ship sewage systems” in MSC/Circ.648 (MEPC 53/16 contains MSC/Circ. 648 and can be downloaded from the IMO Docs Archive)

In addition, septic conditions of sewage treatment plant influent, represented by low oxidation-reduction potential (ORP), can negatively impact on the optimum coagulation and flocculation process setup and the efficacy of biological treatment processes, affecting treatment stability and performance.

FEEDING A SEWAGE TREATMENT PLANT FROM A SEWAGE MIXING TANK

Having an aerated mixing tank of adequate capacity is just a start. For a sewage treatment plant to perform, its influent flow (Qi) must not exceed its designed and certified average and maximum hydraulic capacities. The feed system connecting the mixing tank and the sewage treatment plant, which consists of feed pumps, level sensors, and control logic, must be suitably designed and commissioned for each installation.

An sewage treatment plant designed to take an even influent flow (peak factor = 1) is often fed by an integrated progressive cavity pump completed with suitable control logic as part of the approved sewage treatment plant. This, albeit at a higher cost, ensures the sewage treatment plant can perform within its approved hydraulic capacity.

However, not all sewage treatment plants are designed, approved, or supplied with such considerations in mind. A centrifugal pump can deliver an instantaneous flow of 1~2 magnitudes higher than the certified maximum sewage treatment plant hydraulic capacity. A mixing tank can be turned into a sewage reservoir, flushing the sewage treatment plant with huge batches on automatic level controls. These feed regimes do not represent the type of approved conditions of the sewage treatment plants and constitute serious non-conformities that rid the sewage treatment plants of the possibility to perform.

A sewage mixing tank itself does not necessarily warrant satisfactory sewage treatment plant influent conditions. It needs technical know-how.

FUTURE-PROOFING AGAINST THE EXISTING AND NEW REQUIREMENTS

Sewage treatment plant influent flow (Qi) is essential for the compliant operation of dilution machines that are certified with a Qi/Qe <1. Likewise, influent flow (Qi) and concentrations (Ci) are essential for sewage treatment plants approved with percentage nutrient removal standards.

The current IMO guideline for sewage treatment plants, IMO MEPC.227(64), also requests that ‘the sewage treatment plant influent (Qi and Ci) should be assessed without the contribution of any return liquors, wash water, or recirculates, etc., generated from the sewage treatment plant’. This means that a mixing tank should not receive sewage treatment plant sludge or wash water whenever assessments of Qi and CI are required, whether it is during a sewage treatment plant type-approval test or compliant operations.

These existing requirements are poorly implemented or enforced, leading to multiple type-approved non-conformities that are left unacknowledged or corrected. The situation might change in the near future when the IMO’s MARPOL Annex IV is under revision to ‘confirm the lifetime performance of the sewage treatment plants’.

An aerated mixing tank is set to become increasingly important in the compliant operation of sewage treatment plants on ships. Regulatory and technical know-how are essential to getting it right from now on.

Continue reading

by Dipl. Ing. Olaf Hansen, Head of Technical Department at HAMANN AG

During the last months, the industry of wastewater and sewage treatment technology has seen some notable and partly confusing actions regarding the certification of products. Due to increasing pressure from both the industry and the legislative side, certificates for some sewage treatment plants on the market have been suspended or withdrawn by certifying bodies. This indicates that the certification system needs to be revised. A group of industry players – we are among them – has issued a call for action to improve the guidelines for the certification of sewage treatment plants.

Fortunately, it is the intention of many ship owners to reduce the environmental impact of their ships or just aiming to comply with existing regulations. As a matter of fact, these owners are exposed to a high financial risk when the sewage treatment systems on board do not comply with required effluent standards.

Public authorities around the world are getting more and more sensitive to marine pollution by sewage from ships. A recent example is the Maritime and Port Authority of Singapore (MPA Singapore) which has just issued a circular to all owners, managers and masters of Singapore-registered ships in which they point out that malfunctions of sewage treatment plants have been the most frequent reason for ships to be detained by Port State Control in the year 2019 so far.

If a certificate of an operating plant gets withdrawn because it does not meet the required effluent standards, the ship owner might be forced to have it replaced by another plant with a valid certificate and/or to pay a significant fine.

Considering this, we encourage all ship owners and shipyards to look beyond the certificates and to challenge us – the manufacturers – to deliver sewage treatment systems that perform in terms of effluent quality, ease of operation and reliability. We suggest to broaden the perspective and to take a look at what a sewage management system actually is – or should be.

Sewage management is more than having a sewage treatment plant. A sewage management system needs to be designed properly: From sewage collection via sludge handling to effluent discharge. Sewage treatment plants, black and grey water lines, grease separators, system separators preventing microbes from spreading against the flow from waste water lines into fresh water systems, holding tanks, tank aerations, transfer systems, sludge tanks, sludge processing facilities and performance monitoring are all part of the system. Off course, every project is different. Therefore a sewage management system has to be customized to cover the specific conditions on board.

There are a few key factors that influence how well a sewage treatment system performs. First and foremost, food waste and sewage lines strictly have to be separated. Food waste has a much higher BOD₅ (Biological Oxygen Demand) value than raw sewage and therefore heavily increases the organic load on the sewage treatment plant. Grey water from galleys needs to run through a properly designed and dimensioned grease separator before being stored in a holding tank. Grease from galleys not only plugs up pipes and sensors, but also catalyzes the build-up process of hydrogen sulfide which is a toxic and extremely smelly gas inside the holding tanks. In the worst case, sulphuric acid may be formed inside the holding tanks which then attacks the piping and leads to a degradation of the effluent values.

Last but not least, the organic load of the sewage treatment plant should be monitored and controlled in order to maintain the organic design-load which was certified. For example, IMO resolution MEPC.227(64) requires a sewage treatment plant to produce the defined effluent values based on a TSS (Total Suspended Solids) value of ≥ 500 mg/l in the influent. Our sewage treatment plants are designed within these parameters but with a margin allowing for varying load situations. Operating the plant for a longer period above this margin leads to reduced treatment effectiveness and therefore a degradation of the effluent quality. In extreme cases, overloading can lead to a complete breakdown of the plant.

For us as a manufacturer it is evident that we cannot leave the planning and design of a sewage treatment system to the customers and just sell them sewage treatment plants. An important part of our business is consulting. The earlier we get involved in a project, the better the results can be. We often do workshops with ship design companies and naval architects before or at the very beginning of a project to improve the sewage management system as a whole. During the detail construction we closely work together with the shipyard engineers on creating the best possible conditions on board in order to operate our systems properly.

Sewage management is more than having a sewage treatment plant. It’s a properly designed wastewater management system. But even the best system is useless if operation and maintenance overtaxes the crew on board. It makes no sense to expect a ship’s crew to have expert knowledge in the field of sewage treatment like staff of most land based sewage treatment facilities have. Marine sewage treatment plants should be operated safely without years of prior training. We incorporate this aspect in the design of our systems and put a lot of effort in the automation of routine operations, high usability and low maintenance requirements. Only then, a well-designed sewage management system performs well.

Continue reading