HoldingTank

Holding Tanks

Requirements

Regulations require holding tanks to be an integral part of the hull, or similarly securely attached. If attached, a neoprene buffer should be used between the tank and any chafing points. Tanks must have an external level indicator, and a ventilation system fitted with a flame screen. The level indicator must show when the tank is ¾ full (USA regulation 33 CFR 159.83). Consider an alarm when 75%percent full. If the output pipe is fitted with “Y” valves, to switch between pump-out and overboard discharge, the “Y” valve must be padlocked in Canadian waters so that sewage cannot be pumped overboard. Similar restrictions apply in USA No Discharge areas.

Capacity

The size of the tank depends on the boat'sSecond intended usage (day trips, weekend or extended cruises, etc.). Assume three to five galGallon (11-19 Llitre) per person day. The size of tank required is thus:

Size = k(P*DDisplacement, Depth of ship*E)
Where:
KKelvin, SI unit of thermodynamic temperature is 1.4 (the inverse of 75% rounded up)
P is the number of people on board
D is the number of days in the cruise
E is gallons or litres per person per day

Therefore, for four people on a 10-day cruise, you would require a holding tank of 168 gal (636 L). For 10 people on a two-day cruise, you would require an 80-gal tank (300 L). Most commercial tanks are no larger than 40 gal (150 L), so larger tanks must be custom constructed.

Construction

Choices of material for a holding tank are:

• Polyethylene
• Fibreglass reinforced plastic
• Metal

Either linear (not cross-linked) polyethylene or fibreglass is a good choice. Aluminium or stainless steel tanks are easy to fabricate in place, and not subject to odour permeation. But urine will corrode any kind of weld within a few years. Therefore metal tanks should be coated inside with a ceramic-filled epoxy. Use 3/16-in steel.

Ceramic-filled epoxy is a two-component, reinforced polymeric com-pound that can be brushed on.

Polyethylene, in 40-gal sizes, should be a minimum of 3/8-inch thick, for structural rigidity and resistance to odour permeation. Keep it out of direct sunlight.

All connections should be through the top of the tank (input, deck discharge, overboard discharge, vents, level indicator, inspection port).

Layout

The holding tank is located central to the toilets and mounted as low as possible, especially for gravity-type marine toilets. This reduces the length of piping needed, and effects on the trim of the boat.

Layout of the holding tank (source unknown

Connections include:

• Intake (one per toilet)
• Deck discharge
• Overboard discharge
• Vents (2)
• Inspection port
• Level indicator

Vents

Vents are essential to release any pressure build-up and to control odours. Vent hose is usually 1-in in diameter. Vent filters can be put inline to reduce odour, but it is better to increase ventilation of the tank.

Vent hull fittings are a straight open type made of chrome-plated zinc, brass, bronze, stainless steel or plastic. The vent line should be as short as possible in a straight horizontal line. Avoid sags, arches and bends.

Overfilling the tank will force sewage into the vent line, building up pressure in the system. For this reason, as described in Capacity, above, the size of the tank is such that at maximum usage it is only 75% full, i.e., there is a 25% buffer.

Vented Loops (Siphon Breaks)

Any water intake valves for toilets using seawater, and any discharge valves should be fitted with a siphon break. AAmpere (amp), SI unit of electrical current siphon break is a loop in the line above the water level. It is fitted with a valve at the highest point so that air can be drawn into the line.

The valve is one-way, drawing air into the line but preventing water or sewage from escaping. Larger valves are less susceptible to clogging. Siphon breaks are typically made of cast bronze or glass-reinforced plastic, which is not affected by corrosion.

Deck Fittings

The USA federal standard for a deck fitting for pump out is 1.5-in (38mm) inside diameter pipe thread. The plug should be clearly and permanently labelled "Waste". Consider a black plug for waste, a white plug for water and an orange plug for diesel.

Through Hulls

Toilet intake and overboard discharge lines should be equipped with sea cocks (valves) to stop the inflow of seawater in the event of a hose failure. Sea cocks have a lever-type handle that clearly shows whether it is open or closed. All sea cocks should be located with easy access, and oriented so that the open position is vertical.

Sea cocks and through-hulls are made of bronze or glass reinforced plastic. Both can be used successfully for toilet intake or overboard discharge lines. Bronze sea cocks and both bronze and glass reinforced plastic ball valves may be used for toilet intake or overboard discharge lines. It’s generally a good idea, however, to fit plastic valves on reinforced plastic through-hulls. Either glass reinforced plastic or bronze valves may be fitted to a bronze through-hull.

The use of gate valves, which are made of brass, should be avoided because they corrode rapidly and their stems tend to break.

Piping/Hoses

There is some disagreement about the type of conduit to use for plumbing. Many government marine web sites recommend rigid plastic PVCPoly Vinyl Chloride pipe. It is highly impermeable and will not corrode. It is easy to cut and install. Chief disadvantage is that it is rigid and susceptible to damage from flexing. If you use rigid pipe, use Schedule 80 PVC or ABSAmerican Bureau of Shipping - also AB, together with flexible hose connections to prevent stress on the pipe. Don’t use Schedule 70 PVC.

A preferred solution is flexible sanitation hose, compounded to minimize permeation. This is typically made of flexible PVC or rubber and labelled sanitation hose. Thicker walls increase the resistance to odour permeation. Disadvantages are a shorter life, susceptibility to permeation and sagging.

Sanitation hose should be flexible smooth-walled PVC with an inner diameter of 1.5-in (38mm) or 1.875-in. Be aware that standard fittings are 1.5-in. Use two 100% stainless-steel clamps at every connection. Typical hose vendors are Dometic, Raritan, HoseXPress, Tek-Tanks and Trident Marine.

"Y" Valves

"Y" valves are used to mechanically direct waste overboard, into the holding tank or to a deck pump-out fitting. Regulations require that "Y" valves are padlocked in no-discharge areas, so make sure you can easily access it, readily identify the closed position, and that it is lockable. Most "Y" valves used in sewage systems are made of plastic; however, bronze may also be used. Key attributes of a good "Y" valve include corrosion resistance, fully operating ports for minimal resistance to flow, ease of disassembly for maintenance, and a sturdy handle for positive stops