Sustainable, safe and economically viable
Juan Gonzalez, Ingemar Karlsson of Kemira outline advances in treatment to produce a high quality product
The sludge from KREPRO has a high-energy content (8000 MJ per ton dewatered sludge) equal to woodchips. As the phosphorus is removed beforehand, the sludge can be used for energy production without losing this valuable resource.
Table 1 illustrates heavy metals and toxic organic substances in mg per kg phosphorus in digested sludge and the KREPRO phosphorus fraction. Iron phosphate has been shown to have physiochemical properties, making it a very promising raw material for fertiliser applications. It is not water soluble but is 100% soluble in ammonium citrate (pH7) and 30% in citric acid (pH2). This means that there are many soil conditions in which Krepro phosphate is suitable for agriculture. In certain growing conditions Krepro phosphate has prevented nutrient leakage to surrounding waterways, especially for plants with a long growth period.
|mg/kg P||Digested Conventional Sludge||KREPRO phosphate|
Table 1: KREPRO phosphate makes for high quality farmland fertiliser
How safe is sewage sludge
Most urban areas in the western world are connected to advanced sewage treatment with stringent effluent standards. The sludge from sewage works is, if spread on farmland, regulated by strict quality requirements, e.g. heavy metals and organic toxins must be low. But although popular with farmers, because of the valuable phosphorus content, the spreading of sewage sludge on farmland is always under scrutiny.
For instance, in Sweden there have been alarming reports dealing with contamination caused by silver, PCBs and brominated flame-retardants. The Swedish food industry does not accept raw materials produced from farmland contaminated with sewage sludge, and the Farmers' Association has recommended their members to avoid sludge spreading on farmland, no matter how clean this sludge is. Many other countries are likely to express the same concerns in the near future. Disposal of organic waste is likely to be prohibited or strictly limited after 2005.
The remaining options for sludge are then controlled landfill, composting or incineration. All are expensive and volume reduction is important to reduce costs. Current sludge treatment processes are designed to dilute the sludge or reduce its volume. Composting or mixing the sludge with lime only produces a diluting effect. Volume reduction can be achieved by hydrolysis, by improved dewatering or by drying.
If sludge is disposed to landfill or incinerated, it is no longer possible to recycle the phosphorus. Since new techniques for phosphorus recovery are now being developed, the Swedish Environmental Protection Agency recommends that the new European Sludge Directive should seek not only to promote various conventional ways of using sewage sludge, but also to open the door for alternative ways of utilising the phosphorus in the sewage sludge.
Volume reduction of the dewatered sludge can decrease the cost for landfill and incineration but to decrease the water content is costly. Using more polymers in the dewatering process does not significantly reduce the water content. Sludge drying demands energy (12,000 MJ per ton of DS) and is expensive.
The organic content in raw sludge is about 70% of TS. In digested sludge the organic content is reduced to about 50% but the inorganic part is not affected and represents the rest of the sludge. If raw sludge is hydrolysed by thermal treatment followed by digestion, the organic content can be reduced to 40% but the inorganic part remains as solids, and dangerous substances like heavy metals are now in a higher concentration.
The KREPRO process is based on a new separation technology able to separate valuable products from municipal wastewater sludge. Four main products are recovered from the sludge: Fibre for energy production; Phosphate fraction for fertiliser; Precipitant for recycling in sewage plants; Carbon for nitrogen removal.
KREPRO is a modular process which can be accommodated to local conditions, with options to achieve one or several of the following targets: volume reduction (always); phosphate production; nitrogen removal; heavy metals removal; precipitant recovery. Both digested and raw sludge can be treated. The process is continuous, and can be divided into five main steps:
- Acidic thermal hydrolysis with heat recovery
- Fibre fraction separation
- Phosphorus precipitation
- Phosphate separation
- Recycling of precipitant and car carbon source
The dewatered sludge is diluted to about 10% DS with heat exchanged sewage effluent. Sulphuric acid is added to pH 1,5. Heavy metals, other metals and the phosphorous are partly dissolved by this treatment. The organic suspended material is to a low degree dissolved.
The acidified sludge is heated to 140°C in a pressure vessel for 60 minutes, and 30-40% of the suspended organic matter is hydrolysed into a readily biodegradable liquid. The inorganic compounds are now in a dissolved state, with exception of grit and sand. The undissolved organic matter, mainly fibres, now very easy to dewater, is separated in a centrifuge to a dry solid content of about 50%. The volume reduction compared to conventional dewatered digested sludge is more than 75%.
The energy content in this fraction is high, 8MJ/Kg, equal to that of wood chips, and consequently can be used for energy production. The heavy metals can be separated together with the fibre or later in the process. The supernatant from the separation of the fibre fraction contains the inorganic substances, from which a part of the ferro-ions are oxidised to ferri-ions and the phosphorus can be precipitated as a ferri-phosphate at a pH below 3.
This phosphate fraction is separated by centrifuge. The content of heavy metals
and organic toxic substances is negligible, and can be used directly and safely
as a fertiliser on farmland. The liquid phase from the phosphate separation
contains the precipitant, the dissolved organic matter and the nitrogen. The
relation between organic matter and nitrogen (COD:N ratio) is 10:1, favourable
for biological nitrogen removal. This liquid is recycled in the sewage plant
for nutrient removal.