MAP Technical Reports Series No. 106 UNEP

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presenting the greatest risk are those connected with food processing, such as dairies,
slaughterhouses, canneries, breweries, etc., and manufacturing industries, such as paper and
leather products. In these instances, all waste should be treated before being discharged into
any body of water.
10.2 Reduction of nutrient levels
10.2.1 Sewage treatment plants
There are various methods of purifying raw sewage from urban and industrial
complexes. According to the level of purification to be achieved, one distinguishes between
primary (simple gross sedimentation and clarification), secondary (biological treatment, oxidation
and clarification), and advanced or tertiary treatment to substantially reduce phosphorus and
nitrogen levels. Standard biological treatment will reduce phosphorus and nitrogen by some 20
or 25%. In the case of phosphorus greater reduction is achieved by chemical precipitation,
adding salts of aluminium and iron at certain stages of the treatment process. With to-day
standard technology as much as 90% reduction can be easily achieved; using advanced
technology another 5 to 9% can be gained, but treatment costs will substantially increase.
Nitrogen can be reduced by biological methods based on processes that occur
spontaneously in nature, namely nitrification and denitrification. The methods most widely used
consist in a sequential process chain (aerobic-anoxic, anoxic-aerobic, alternated aeration),
designed to modify the oxidation state of nitrogen to obtain its release ultimately in volatile form.
Nitrogen reduction technology is relatively costly, and only warranted, where nitrogen load from
urban areas makes out a substantial fraction of the total nitrogen load.
A somewhat different but corresponding biological method has also been developed
for phosphorus, but the technique has not generally been adopted yet.
10.2.2 Other forms of purification
In addition to methods of nutrient reduction in conventional treatment plants, there are
other methods available (Merrill, 1991), such as phytopurification, lagooning and fertirrigation,
to cut down the nutrient load. These are usually applicable only downstream of the plant.
Methods of this kind have been tried in many parts of the world. The first two are based on the
capacity of growing plant biomass (either naturally growing or introduced as in the case of water
lilies) to absorb large amounts of nutrients, and thus to abstract them from the body of water.
Biomass grown in this way in lagoons, like the macroalgae (ulvaceae) that often tend
to amass in their relatively still and/or shallow waters, must be removed periodically to minimize
the risk of new release of nutrients that would otherwise accumulate in the basin by
mineralization, and hence upset the rational for the procedure. Among the main drawback which
frequently disallow the adoption of such a solution are discernible in the costs of transporting the
large volumes of biomass collected, the lack of suitable storage sites, and the lack of
opportunities to rationally utilize the biomass stored for other purposes.
Composting and utilization for soil conditioning is limited by reason of the high salt
content of marine macroalgae. Biomass of e.g., water lilies has been utilized for feed of hogs,
biogas, and paper. However, this otherwise highly efficient water plant for removing nutrients