Re: Constructed Wetlands

Not too surprisingly the designers of artificial wetlands are usually
engineers and only consider the aquatic plants selection as an
afterthought.They seem to be more concerned with flow rates and stocking
densities etc

I agree with Neale that water chemistry and temperature are very important
but since Tilapia are the target species that seems to be covered. I have
looked at several of these projects ( at least the overall plan in most)
and very few seem to have considered the problem posed by the initial
success of the plantings and resultant vigorous growth. No matter which
species you choose eventually the plants have to be continually harvested
to remove the accumulated nutrients stored in them before they start to rot
and return those stored nutrients back to the system and of course reduce
the efficiency of their original purpose.

Accordingly a harvesting proceess ( preferrably automatic) needs to be
designed in from the start. For substrate-anchored species this is usually
either labour intensive or requires expensive machinery. A system based on
floating plants is simplest and cheapest and a spillway design that allows
the sideways pressure of the multiplying plants to forse excess plants out
of the system is close to automatic. The excess plants can be spilled into
perforated containers and used for garden mulch, pigfood ( as part of a
multi-species project) or other secondary purpose.

Many local authorities would require you to have adequate safeguards to
prevent potentially noxious species such as Pistia and "Hyacinth" from
entering natural waterways. I am assuming that the treated water from your
system is being returned to the system and not just being "made safe" to be
drained off to a natural waterway.

Bruce Hansen

email: bhansen at ozemail_com.au
Don't miss the ANGFA web pages at -

<<From: N.Monks at nhm_ac.uk (Neale Monks)
Date: Sat, 21 Dec 1996 13:17:46 +0000
Subject: Re: constructed wetland for Aquaculture

Assuming that your water is above 15-degrees C and well lit, Hygrophila
polysperma and Vallisneria gigantea always seem to be very fast growing,
high nitrate tolerant plants. Both grow well in fast moving water with
minimal dissolved oxygen.

In very hard water, or slightly salty water, Elodea and Egeria do well, but
they tend to need cooler water, say 10 to 15-degrees.

Alernatively, surface plants like water hyacinth and Pistia strationes grow
very quickly, far faster than submerged plants, and are indifferent to
water quality.>>