Odour prevention with the help of a special nitrate solution (Nutriox®) and the subsequent stimulation of nitrate reducing bacteria (NRB) is a well proven concept in sewerage collection systems and other types of waste water applications [1,2]. Biosolids originating from treatment plants are often a source of hydrogen sulphide (H2S), reduced sulphur compounds (RSC), as well as other odorants. Treatment of biosolids with nitrate salts has been done to a far lesser extent and practical experience is limited [3,4]. This paper reports on results from pilot scale testing at the La Crau treatment plant in the south of France and from two full scale installations in Lillehammer and Hokksund, Norway.
At La Crau WWTP, biosolids are stored in open containers up to four days after dewatering, resulting in emission of odours. During a trial, the emission of H2S, RSC and ammonia were monitored with and without treatment with Nitrate.
Nitrate salts were proven to be an effective measure for odour control in wastewater biosolids. Odour was considerably reduced over a period of four days; increased dosage rates may result in longer odour control timeframes.
In the two full-scale cases high H2S concentration in relation to handling and storage of biosolids was observed as a problem.
At Lillehammer WWTP H2S was emitted especially from sludge thickeners, buffer tank and from dewatering. Three Nutriox® dosing systems were installed treating biosolids at the inlet of the sludge thickener, half way down into the cone and into the buffer tank before the dewatering, respectively. H2S concentration was reduced from over 75ppm to zero.
At Hokksund the main odour problem arose during handling of dewatered sludge after it was stored in a silo, especially when loading and unloading trucks. Spraying Nitrate onto the dewatered sludge when leaving the centrifuge before it entered a silo solved the problem. H2S peaks of up to 40 ppm were reduced to values under 5ppm when the silo was emptied.
Dosage of about 0.2 – 0.5% (w/w) into sludge thickener and about 2% (w/w) on dewatered sludge has proven optimal in the cases described. However, dosage of Nitrate depends on the characteristics of the sludge, and the required duration of the effect, and can vary from case to case.
All three cases show that nitrate can be effectively used to reduce odour related to handling and storage of biosolids. There is a demonstrable effect in wet or dewatered sludge. The key parameter, H2S, is reduced in concentration, as are Mercaptans and Ammonia.
Dosing with Nutriox® will make handling of biosolids safer for operators and will reduce odour complaints from the neighbourhood.
KEY WORDS Ammonia, Biosolids, dewatered sludge, hydrogen sulphide, mercaptans, odour, reduced sulphur compounds
