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<pre>Size = k(P*D*E)Where:K is 1.4 (the inverse of 75% rounded up)P is the number of people on boardD is the number of days in the cruiseE is gallons or litres per person per day</pre>= Vents ==
ThereforeVents 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, for four people on but it is better to increase ventilation of the tank. Vent hull fittings are a 10straight open type made of chrome-day cruiseplated zinc, you would require brass, bronze, stainless steel or plastic. The vent line should be as short as possible in a holding straight horizontal line. Avoid sags, arches and bends. Overfilling the tank of 168 gal (636 L)will force sewage into the vent line, building up pressure in the system. For 10 people on a two-day cruisethis reason, you would require an 80-gal as described in [[#Capacity|Capacity]], above, the size of the tank (300 L)is such that at maximum usage it is only 75% full, i. Most commercial tanks are no larger than 40 gal e., there is a 25% buffer. == Vented Loops (150 LSiphon Breaks)== Any water intake valves for toilets using seawater, and any discharge valves should be fitted with a siphon break. A siphon break is a loop in the line above the water level. It is fitted with a valve at the highest point so larger tanks must that air can be custom constructeddrawn 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.
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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% 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's intended usage (day trips, weekend or extended cruises, etc.). Assume three to five gal (11-19 L) per person day. The size of tank required is thus:
<pre>Size = k(P*D*E)
Where:
K 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</pre>
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 ==
All connections should be through the top of the tank (input, deck discharge, overboard discharge, vents, level indicator, inspection port).
== Capacity Layout ==
The size holding tank is located central to the toilets and mounted as low as possible. This reduces the length of the tank depends piping needed, and effects on the trim of the boat's intended usage . Connections include:* Intake (day trips, weekend or extended cruises, etc.one per toilet). Assume three to five gal * Deck discharge* Overboard discharge* Vents (11-19 L2) per person day. The size of tank required is thus:* Inspection port* Level indicator
[[Category:SewageSystems]]