Wärtsilä Hamworthy opts for separate treatment of grey water
Wärtsilä article August 2012
Text: Mr Wei Chen, Manager, R&D, Water Systems, Wärtsilä Hamworthy Ltd.
The article was originally produced for the use of Schiff & Hafen, Marine Engineers Review and Maritime Market.
Leading manufacturer continues to provide certified conventional and advanced wastewater treatment plants, helping thousands of ship owners and operators to meet their pollution obligations
A host of regulations calling for more stringent black and grey water discharge standards has been introduced in recent years urging the shipping industry to react to increasing environmental demands.
Rules on black water discharge, specified in MEPC 159(55) under MARPOL Annex IV, came into force in January 2010. They cut permissible sewage discharge levels of suspended solids from new buildings by more than half, even in open sea, the volume of allowable faecal coliform discharges by more than 70% and biochemical oxygen demand by 50%. The rules also introduced a particularly low limit of residual chlorine to protect the marine environment and aqua life from the side effects of sewage treatment.
Regional rules vary and are changing quickly. In Alaska, stringent limits regarding cruise ship discharges were introduced in 2000 (33CFR159 sub-part E) with grey water regulated for the first time. High volumes of grey water are produced on cruise ships, often as much as eight to ten times the amount of black water, and conventional sewage treatment technology has struggled. Technology suppliers including Hamworthy quickly came up with solutions such as Advanced Wastewater Treatment (AWT) systems to assist owners and operators to fulfill their environmental obligations.
In 2004, the US Environmental Protection Agency examined AWT systems supplied to four different cruise ships and concluded the technologies were proven. The agency specified the same discharge limits in the National Pollutant Discharge Elimination System Vessel General Permit which regulates the grey water discharges from ships in all US coastal waters. The Alaska Department of Environmental Conservation also branded the proven AWT systems as the best available technology for ships.
It is important to note that the limits specified had already adopted the secondary treatment standards applicable for land based treatment facilities. Shipboard AWT systems produce effluent to a much higher quality than that of the sewage treatment works serving local towns and cities.
In 2007, the Department’s Wastewater Discharge General Permit for large cruise ships included additional discharge limits including ammonia and metals; these were revised again in early 2010. Some of the standards far exceed the equivalent limits in other industries. The Department will continue to review the General Permit on a three-year cycle, while searching for viable technologies to reach the ultimate goal of Alaska Water Quality Standards, which includes limits such as the maximum dissolved copper of 3.1 microgram/L.
In Europe, the Baltic Sea may soon become the first ever Special Area under MARPOL Annex IV. The Helsinki Commission, HELCOM, which works to protect the marine environment of the Baltic Sea from all sources of pollution through intergovernmental co-operation between Denmark, Estonia, the European Community, Finland, Germany, Latvia, Lithuania, Poland, Russia and Sweden, has proposed a set of nutrient discharge standards for passenger ship sewage discharge to combat the eutrophication issue. The limits include total nitrogen of less than 20 mgN/l, or a removal rate of more than 70%, and total phosphorus of less than 1 mgP/l, or a removal rate of more than 80%. These limits exceed the current and future limits of the equivalent land-based discharges in the Baltic States. Similar initiatives may arise in other enclosed/semi-enclosed sea areas.
Some international and regional rules for black and grey water discharges
Sub part E
|Applicable region||IMO water||Alaska||US water||Alaska||Alaska||Baltic Sea|
|Total Nitrogen||mgN/l||20, or 70%|
|Total Phosphorus||mgP/l||1, or 80%|
Rapidly changing regulatory developments have imposed significant challenges on cruise ship design and operation. Increasingly strict requirements have stretched the boundary of proven AWT systems technology, commercial viability and environmental sustainability. With a lack of proven technologies on the market, ship yards are considering a ‘future proofing’ solution through the provision of additional holding tanks for the treated and untreated black and grey water. ‘Holding’ waste involves holding wastewater in special areas, treated or untreated. The waste is then discharged outside 12 nautical miles from shore, or off loaded to a port reception facility.
Such operational practices are already evident in Alaska though many ships opt out of the Alaska discharge permit completely, with the result that vessels have to use greater levels of fuel - thereby creating far greater carbon emissions – in order to reach deeper discharge areas. In 2009, at least 11 cruise ships, with a total of 138 Alaska visits, chose not to discharge in Alaskan waters. For 2011, it has increased to 166 visits.
Hamworthy continues to invest in product development and innovation in order to help clients sustain their compliance operations with minimum environmental impact. The company has carried out detailed surveys on a number of cruise ships to examine the characteristics of black water and various grey waters. It has found that black water contains the majority of the nitrogen and phosphorus load in ship discharges, with concentrations five to ten times higher than typical municipal sewage.
Sewage treatment technologies such as nitrification and denitrification for ammonia and total nitrogen reduction are well developed for land based applications. Hamworthy carried out a year-long cruise ship trial using biological nutrient removal technology on mixed black and grey water treatment. It became apparent that the shipboard solution must be simpler, more robust, environmentally sustainable, and most of all, designed to suit unique shipboard conditions. Furthermore, it was clear that the solution must also be flexible enough to satisfy increasingly diversified regional requirements. One solution that facilitates such goals is the prevention of mixing extremely nutrient-rich black water with the grey water.
Table 2. Typical wastewater characteristics
|Waste Streams||Municipal sewage||Shipboard (cruise)|
|Black water||Grey water|
|Ammonia (mgN/l)||30 – 50||400-1000||1-10|
|Flow (L/person/d)||150 - 200||12-25||
120 – 250
Working closely with a major cruise company, Hamworthy carried out additional extensive shipboard trials and developed the ‘separate grey water treatment configuration’.
In this configuration, grey water is treated separately to satisfy the most stringent requirements such as Alaska GP stationary discharge limits and the Baltic Sea nutrient discharge limits. In doing so, treated grey water can be discharged directly overboard in special areas. Because grey water volume is often eight to ten times that of black water volume, this solution significantly reduces the holding tank requirements and the carbon emissions associated with carrying grey water up and down special areas.
On the other hand, black water is treated to MEPC159(55), and 33 CRF159 sub-part E requirements for discharging outside the special areas or off-loading to port reception facilities. Under the HELCOM proposal, port reception facilities will receive wastewater discharges from passenger ships under a ‘no special fee’ arrangement with the port authorities.
This solution marks a significant contribution to the environmental challenges faced by the cruise industry, as it maximises AWT systems potential, minimises the holding requirement and emissions and facilitates efforts by the port authorities.
Waste segregation is not a new concept, and is understood and practiced by many who are environmentally conscious. Hamworthy’s separate grey water treatment configuration formulates a unique shipboard black and grey water management system, thanks to the unique features of the cross-flow side-stream MBR technology. These features enable black and grey water to be managed by the identical MBR plants of the existing Hamworthy design, and are of similar operational and control regimes.
In normal circumstances, black water treatment is essential while grey water treatment is not required. In this case, the MBR for grey water treatment is in effect a stand-by plant. In special areas such as Alaska and the Baltic Sea it is more essential to reduce grey water holding requirements and in this case, the MBR for black water treatment could be used for grey water treatment when needed. The change-over of duties and treatment could be achieved in less than a day.
When the mixed black and grey water is treated by nitrification and denitrification to the same standards, matching the separate grey water treatment configuration, the bioreactor volume increases by 50% to 100%, energy consumption increases by 35% to 40% and chemical consumption for phosphorus rises by 200%.
Hamworthy’s solution secures treatment availability, lowering energy requirements, consumables, and most importantly, it sustains the simple treatment chain and low skill requirement for reliable and robust compliance operation in sensitive areas.
Figure 1. Diagram of Separate Grey Water Treatment Configuration
The separate grey water treatment configuration proved extremely successful on two cruise ships during the 2010 Alaska Season. The performance of the treated grey water exceeded the most stringent stationary discharge limits in the General Permit (Figure 2) and the concept has been well received by a number of cruise companies and ship yards.
Figure 2. Performance of the treated grey water comparing to that of mixed wastewater.