Some 20 million people board cruise ships
every year. While they may return to land
with fond memories of umbrella drinks,
tanning on a wooden deck or cuisine
dining, they leave a lot at sea. The US
Environmental Protection Agency (EPA)
estimates that a single 3 000-person
cruise ship can discharge more than half
a million litres of sewage into the ocean
per week. Discharges of black and grey
water generated and discharged during
normal ship operations is regulated
Water supply in Venice
through the ages
Venice is a city built on water with
everything transported on the world-
famous canals, but the water in
the lagoon is too salty to drink. The
drinkable water problem was of primary
importance in Venice from the first day
of its existence and lasted throughout its
millennial history. It is a fascinating story.
When the people of the mainland,
under the pressure of barbarian
invasions, took refuge on the lagoon
islands, they may have started by taking
fresh water from rivers entering the
Water Sewage & Effluent July/August 2017
35
Sewage disposal from
cruise ships
using 600 tons of water every day.
Although some potable water is bought
at ports, most of the water is produced
from seawater on-board through two
processes: distillation by using excess
heat from the huge engines, and reverse
osmosis. After desalination, the water is
passed through a mineralisation plant,
to add the necessary healthy minerals
removed by desalination.
lagoon. But for centuries, potable water
was supplied by a traditional rainwater
collection system built in the city
squares: it consisted of a well head and
an underground cistern, filled with clean
sand, with a waterproof layer of clay all
around that served as a barrier against
the infiltration of saltwater (Figure 1).
The rainwater penetrated the ground
by means of collectors positioned around
the well, located at slightly lower levels
than the rest of the square (or campo).
It filtered through the sand down to the
waterproof clay bottom of the cistern.
The well shaft, which was waterproofed
by a layer of clay spread along its entire
depth, filled up from below with the
collected water, which had been purified
as it drained through the sand. The water
was drawn with buckets.
At the beginning of 1300, there were
100 wells in the city already. Further
measures were taken to increase the
number of wells, for example, religious
organisations that constructed wells
inside the monasteries were largely
subsidised by the State if the wells
were left free to be used by all citizens.
Stringent measures were adopted to
avoid waste in consumption: the parish
priests held the keys of the wells, with
the task of opening them only twice a
day at the sound of the ‘well bell’. The
well heads (vera da pozzo), still visible
across the city, are the only exposed
part of the century-old well system in
Venice. The well head served as a cap
on the well to prevent debris from falling
into the well and contaminating the
freshwater supply. Strict laws governed
the drawing of water as well as the
purity of the font. It was absolutely
forbidden to approach the well with
soiled hands or a dirty container.
As the population increased
and business grew, the cisterns and
wells could no longer cope with the
increasing water demand. The Senate
Venice, a water wonderland.
internationally through Annex IV of
the International Convention for the
Prevention of Pollution from Ships
(MARPOL). It is generally considered that
on the high seas, the oceans are capable
of assimilating and dealing with raw
sewage through natural bacterial action.
This regulation of MARPOL
stipulates that treated sewage can be
released into the ocean on condition
that the ship is more than three
nautical miles from the nearest land or,
in the case of untreated sewage, at a
distance more than 12 nautical miles.
The regulation also stipulates that
such sewage being held in storage
tanks should not be discharged
‘instantaneously’, but at a ‘moderate
rate when the ship is proceeding at no
less than four knots’. The regulation
further specifies that the ship must
have in operation ‘an approved sewage
treatment plant that has been certified
by the Administration to meet the
operational requirements’. Additionally,
the ‘effluent shall not produce visible
floating solids nor cause discoloration
of the surrounding water’. Governments
are obliged to ensure the provision of
adequate reception facilities at ports
and terminals for the reception of
sewage, without causing delay to ships.
Figure 1: Diagram of Venetian water cistern and well.