NUMBER TWENTY NINE - OCTOBER 1998
NUMBER TWENTY NINE - OCTOBER 1998
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Box 5:
options for the control of nutrient loads(a) External diffuse loads
(i) Minimal cultivation of land, contour ploughing, parsimonious fertilisation. Effective for N and P.
(ii) Buffer strips or more extensive wetlands to intercept water coming from agricultural land. Very effective for N, less so for P.
(iii) Urban hygiene. Burial of pet faeces, avoidance of car washing on sealed surfaces as opposed to absorbent soil, composting of wastes, not laundering part-loads in washing machines. Probably has small absolute effect on N and P loads but heightens public awareness and paves the way for more significant measures
(iv) Precipitation of P with iron salts in the inflows. Expensive in chemicals, leads to unsightly iron deposits but can be effective for P though undesirable in sites of conservation or amenity importance.
(b) Point source loads
(i) Waste water treatment. Effective. Relatively inexpensive for P, more expensive for N. Can also cope with perceived problems of detergent phosphate.
(ii) Waste water diversion to the sea. Expensive but very effective for N and P. If effluent flows are significant contributors to the total inflow, then flushing rates will be reduced and this may have negative effects. Transfers the problem rather than solving it.
(c) Internal loads
(i) Sediment sealing with fly ash or alum salts or oxidants. Effective for P in the short term, but not consistent with conservation objectives.
(ii) Sediment removal. Very expensive and often ineffective as phosphorus continues to be released from remaining sediment.
(iii) Removal of plant biomass. Inconsistent with conservation objectives and largely ineffective unless carried out over many years because of the large stores contained in the sediment and recycling to the roots and rhizomes.
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