What you need to know about a yacht's black water waste tank
Jun 08, 2023
It is now mandatory in most countries to make provision for sewage to be pumped ashore rather than disposed of at sea. So it may be time to install a black water waste holding tank, writes Duncan Kent
Prior to the new millennium, little thought was given to waste disposal from a vessel. Anything you didn’t want on board invariably went straight over the side, including human waste. Thankfully, these days it’s a different matter altogether. Rubbish is widely recycled worldwide, so keeping it on board until going ashore is commonplace, even when ocean sailing. However, black water (sewage) disposal is not such a simple problem to solve, especially on smaller vessels, and it’s of particular concern when you are coastal cruising, anchoring or mooring close to the shore.
Given the increasing popularity of many northern European, Mediterranean, Pacific and Caribbean anchorages today, I’m convinced no responsible yacht owner would be against banning the direct discharge of black water within at least three miles of the coast. Nothing spoils an early morning dip in crystal clear water more than coming across an unmentionable floating object!
At the time of writing, there are still no mandatory regulations concerning black water disposal at sea from privately owned leisure craft in UK waters, apart from the legal requirement to provide pump-out facilities on inland waterways, but the situation is rapidly changing as other countries update their rules.
Since 2006, the EU Recreational Craft Directive (RCD) has required all new vessels to ‘have provision for a holding tank to be fitted’, although that isn’t the same as actually having a holding tank fitted. Besides, this only applies to new vessels and simply requires a ‘diverter valve’ to be installed into the discharge pipe to enable the waste to be piped into a holding tank.
That said, each country has its own rules about this and some are far stricter than others. For instance, Norway has banned the discharge of sewage within 300m of its coastline, whereas Germany insists you’re 12 nautical miles away before you even think about it. And if you’re even spotted with bubbles emanating from your boat in a Turkish marina you could be fined massively and possibly even imprisoned!
Generally, a holding tank doesn’t necessarily have to be permanently installed (although thankfully most boatbuilders today do).
UK black water regulations currently only apply to inland waters
In theory you could simply divert the waste into a freestanding jerry can or similar for disposal ashore, but this could end up being very unpleasant in rough seas and awkward to empty ashore without considerable mess.
Many boatbuilders that do install them from new, however, often only do so for a single head, leaving others on larger, multi-head yachts to be effectively unusable in prohibited sea areas.
The most common setup in new production boats is to have all the waste go directly into the holding tank, which will have a pipe and deck fitting at the top for pumping out shoreside, like most inland waterways craft have. Seagoing vessels, however, will also have a large diameter outlet at the bottom of the tank, complete with a seacock to allow the waste to gravity-drain overboard when outside the local limit.
This isn’t always the easiest to provide if retrofitting a system, though, as hull and deck fittings ideally need to be aligned (for rodding if necessary) and clear of any other fittings. For this reason, some boat owners simply incorporate a diverter valve into the existing outlet hose from the toilet, allowing the waste to either be pumped directly into the sea or alternatively into a remotely positioned holding tank.
Quick switch: send waste overboard or into a tank with a diverter valve
If this tank only has a deck plate, when the tank is full you are compelled to find a pump-out station, rather than simply sail offshore to empty it.
Tank size: The size of the holding tank will usually be a compromise between the ideal capacity and the space available in a convenient locker.
It should be at least 20 litres capacity, but ideally you should allow for around five litres per person/per day for two days. This means a cruiser, regularly used by a couple plus two guests, should work on a minimum capacity of around 40 litres. Too small and you’ll need to empty it too often; too large and it will add considerable extra weight and use up valuable space.
Location: Most choose to utilise an under-bunk stowage bin as the location for a remote tank, although some boats might have enough room inside the heads. Ideally, it should be positioned as close to the toilet as possible in order to keep hose runs short, but not so close as to force tight bends into any of the pipework. Bear in mind that the longer the inlet hose, the more water you will need to pump down it to clear the waste.
Thought must also be given as to the accessibility of any pumps or diverter valves incorporated into the system and the ease with which all areas can be reached for seasonal cleaning or the clearance of the almost inevitable blockage.
A typical holding tank installation
There are numerous companies producing tanks capable of storing black water. These tanks can be manufactured from stainless steel, GRP or plastic, the latter being either flexible or rigid.Steel tanks are excellent in many respects, but sooner or later their welded seams will begin to rust, expedited by the corrosiveness of the waste. GRP is lighter than steel and almost as odour-free, provided the moulding is perfectly executed and there is no trace of porosity. By far the easiest material, especially for smaller craft, is thick-walled plastic.
Retailers sell a bewildering range of plastic tanks, even some multi-purpose tanks supposedly fit for fuel, water or waste. The truth is many aren’t up to the job so care must be taken when selecting one to make sure it’s suitable for black water.
Dometic’s 15HTS polyethylene holding tank has 6.3mm walls and costs around £600
It is often difficult to spot the difference between a polyethylene (PE) tank (for fuel, fresh and grey water) and a polypropylene (for black water), but the latter is more resistant to corrosion and odour permeation. Wall thickness also needs to be at least 6mm (1/4in), preferably 8mm (3/8in).
A flexible tank can sometimes solve a tight space problem, but in order to avoid the bladder chafing through on a sharp edge, a smooth-sided box must first be built from plywood and GRP. This creates work and limits overall capacity, so in most cases a rigid tank will prove easier to install.
Choosing a standard tank, many of which are available off the shelf in odd shapes and sizes to fit bow and bilge lockers, will save you money over having a custom tank manufactured to your own specifications.
Modern yachts use gravity tanks to discharge waste
Whether you decide to go electric or manual, a tank discharge pump must be man enough for the job. It’s no use using 38mm (1½in) pipes, connectors, and hull fittings if the pump only has 25mm (1in) inlet/outlet spigots. Most manufacturers produce a pump specifically designed for the purpose, usually a large-bore, high-capacity diaphragm device that can be operated manually or electrically, with an integral, non-return valve in its outlet.
It’s also possible to buy and fit 12V macerator pumps that grind up the waste while pumping it out. Their advantage is that a slightly smaller diameter outlet hose can often be used, and the chance of a blockage is reduced. They do, however, use more water than a manual pump, thereby filling the holding tank more quickly.
In modern boats, gravity is usually used to discharge the tank overboard. Although to avoid the danger of blockages with this method the tank outlet and skin fitting need to be large, usually 75-100mm (3-4in) in diameter, which is often not a practical option on a small boat.
New gauges can now indicate the exact level of waste in your holding tank
Diverter valves are widely available from numerous marine plumbing equipment suppliers and are usually designed for bulkhead mounting. For systems that are pumping out with a head higher than 1m, installing a non-return valve into the exit hose will be essential to stop the contents back-feeding. Once again all valves need to be 38mm (1½in) diameter.
This is an important part of the installation, not just for safety reasons, but to ensure no nasty odours can pervade the living quarters. All hoses must be of non-permeable, sanitation grade, flexible PVC and the genuine article will normally be marked accordingly. It’s not particularly cheap, but it’s worth every penny.
As with any sea toilet installation, if the bowl or holding tank are below sea level at any time all pipework connected to a skin fitting must contain a swan-neck bend with an anti-syphon valve at the top. These are freely available in most chandlers for a range of hose diameters. All joints must be double-clipped using good quality, stainless-steel hose clips.
A remote pumped waste tank helps if the layout is awkward
Level gauges: Both electrical and mechanical indicators are available for waste tanks, which differ slightly from the usual water tank sensor in that the sensors are encased to prevent solids from fouling the mechanism.
Gauges are available that indicate the exact level of waste in the tank, but for most installations a simple warning light when the tank is nearly full is adequate.
Venting the tank: The typical odours experienced with poorly installed holding tanks are usually caused by anaerobic bacteria forming in an oxygen-starved environment.
The key to preventing these smells emanating from the tank is ventilation. Supplying oxygen to the tank allows the aerobic bacteria, found naturally in human waste, to carry out the job of breaking itself down. Any gases produced in this process will then be dispersed into the outside air via an overboard vent, preferably installed away from the cockpit area.
Installing a holding tank can be tricky on smaller yachts
Commercially produced tanks are usually supplied with pre-formed inlets and outlets. One of these, positioned as high on the tank as possible, should be the ventilation pipe connection. The vent should ideally exit the tank vertically to prevent any solids blocking the pipe and should then continue to deck level or onto a shell-type skin fitting high on the topsides. The pipe run needs to be kept as short as possible and there should be no tight bends or kinks restricting the airflow. An odour-reducing, active carbon filter can also be installed into the vent pipe without restricting the airflow to the tank.
Water treatment devices, such as the Seasmart sanitation reservoir, are available to kill bacteria in the seawater as it is drawn into the toilet.
The device is plumbed into the inlet hose and contains an environmentally friendly sanitation fluid which, as well as combating incoming seawater bacteria and odour, also claims to reduce the build-up of limescale without having a detrimental effect on the natural aerobic bacteria in the tank.
Simple additives are also available that should eliminate odour from holding tanks and also encourage the breakdown of the waste by natural bacteria. But, whichever additive you choose, don’t be tempted to use domestic toilet cleaner or bleach in either the toilet or the waste holding tank.
US national symbol directing boaters to the nearest pump out station
Although the UK hasn’t, as yet, implemented any regulations regarding black water waste or holding tanks for recreational craft beyond its inland waterways, the situation internationally is now developing fairly rapidly, and unevenly, with some countries taking a much stricter stance than others.
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