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. This reduces the length of piping needed, and effects on the trim of the boat. 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.