CHAP.
LIFE + MANEV
5. 10.
The removal efficiencies of the organic matter and nutrients are usually higher for SSF than for FWS but in SSF an excessive load of suspended solids can cause a rapid clogging. Conversion of wastewater nitrogen to gaseous forms of N is the major mechanism for N removal by CWs, whilst plant uptake and accumulation in the CWs accounted for a limited fraction of the N load. Phosphorus can be oxi-
dised and removed by settling/filtration or removed by plant and microbial adsorption. Predation and natural dieoff of pathogens occurs in constructed wetlands. A system of constructed wetlands for livestock slurry is able to provide adequate performance only if placed at the end of a treatment line that includes upstream other treatments with strong removal of the organic load, suspended solids, nitrogen and phosphorus.
SLURRY PRE-TREATMENT
Figure 5.10.5. Typical layout of a phytodepuration plant, with a pre-treatment, two wetlands and control wells (CRPA, 2006).
PERFORMANCES Main operational information: • Phytodepuration is recommended only for the purification of pre-treated slurry or wastewater • Treated effluent can be discharged both in public sewerage system or in surface watercourse, depending on the residual concentration of pollutants • Treated effluent can be reused for fertigation of crops • Removal rates may vary during the year due to seasonal weather patterns • In some circumstances pollutants might even be released (e.g. extreme rainfalls events) • Expected plant life is at least 10 years • Technology reliability is good for municipal wastewater, not still sufficiently investigated for different kinds of livestock slurry; • In the livestock sector good achievements are limited to the treatment of wastewater from the dairy parlor and pre-treated pig slurry
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• Odor and insects can be a problem, especially in FWS systems • The following loading rates are recommended (EPA, 2000): • BOD5 and TSS
< 70 kg/ha per day
• TKN or NH3
< 3 kg/ha per day
• TP
< 0.2 kg/ha per day
• Hydraulic loading
< 500 m3/ha per day
• Retention time
> 10 days
• The optimal hydraulic load is between 200 and 500 m3/ha day. The risk of having less than 200 m3/ha day when treating slurry is a real risk if the BOD, TSS TKN and TP recommended loading rates are met. Nutrient balance and end-products: • Nutrients removal is affected by the hydraulic retention time (HRT), temperature, physical and chemical properties of the medium, plant type