and considerable time for the gel to set. Mixing studies ()
indicate that a jet nozzle mixer moved about through the tank is an
adequate way of mixing, although such devices are not routinely
built into tankers. Approximately eight hours is generally required
to achieve a gel of modest strength; the gel doubles in strength
after about thirty hours and triples in strength after 130 hours.
No further gain in strength seems to occur after 130 hours.
Goldstein et al. () estimate the cost of gelling oil at
approximately $4.50 per barrel. On a tanker the size of the
Torrey Canyon—modest by comparison with many built in
the last few years—the total cost would be $2.7 million.
Herding
Agents
A herding
agent alters the surface tension of oil-water, water-air, and
oil-air interfaces so as to contract an oil slick rather than
spread it. When a slick attains any modest thickness, gravitational
effects result in spreading forces. Thus herding agents can thicken
an oil film only to a limited extent. As a result, herding agents
are only useful for thin slicks containing small amounts of
oil.
Sorbents
Sorbents may
either absorb or adsorb oil to form a floating mass for later
collection and removal. Sorbents, used either with or without a
containment barrier, are considered by the author to be the only
treating agent that can be used safely and effectively at this
time. Future developments are expected to make them much more
effective, and their potential is a fruitful area for research.
Historically,
the most common method of dealing with relatively small oil spills
has been to use straw as a sorbent. Straw can absorb approximately
five times its own weight in oil. After the straw absorbs as much
oil as possible, it is gathered together and disposed of,
frequently by burning after evaporation of the water. As a result
of a substantial amount of research on sorbents (,,,, and ), the
best sorbent material appears to be reticulated polyurethane foam.
Not only does it absorb about thirty times its own weight in oil,
but the oil can be largely removed by passing the material through
wringers, so that the reticulated foam can be rebroadcast onto the
sea to absorb more oil. Although no full-scale continuous-use
sorbent equipment has been built and used on the sea, Miller ()
estimates that the cost of a unit designed for operation in
protected waters, with a recovery rate of three thousand gallons of
oil per hour, would be $76,480. The three thousand gallon-per-hour
recovery rate is what investigators expect of future sorbent
systems. It is considered to be quite slow for dealing with large
spills at sea, but adequate for many small spills in harbors.
The
effectiveness of a sorbent system is not seriously diminished by
adverse sea conditions. In fact, better results can sometimes be
obtained in the presence of waves than in calm water. If a
mechanical sorbent retrieval system is