By Juan Gonzalez (Kemira Iberica, Kemwater) & Ingemar Karlsson (Kemira Kemi AB, Kemwater)
April 16, 2002 -- A new type of sewage sludge treatment process relies upon new separation technology in a new way.
The Kemwater Recycling Process (KREPRO) for treatment of sewage sludge reduces both the organic and inorganic content of the sludge. The organic content is reduced by about 30% and the inorganic about 70% if digested sludge is treated. All metals and salts are dissolved; the remaining undissolved inorganics are mainly silicates like sand and grit.
In contrast, most sludge treatment technologies used today reduce only some parts of the organic content of the sludge, but nothing of the inorganic part. Half of the dry solids in a digested sludge consist of inorganics.
The process reduces both the organic and inorganic content of the sludge. The organic content is reduced with about 30% and the inorganic with about 70% if digested sludge is treated. All metals and salts are dissolved; the remaining undissolved inorganics are mainly silicates like sand and grit.
The sludge from the process has a high-energy content (8000 MJ per ton dewatered sludge) equal to woodchips. As the phosphorous is removed beforehand, the sludge can be used for energy production without loosing this valuable resource.
Iron phosphates have shown to have physiochemical properties, making it a very promising raw material for fertilizer applications. It is not water soluble but to 100% soluble in ammonium citrate (pH 7) and to 30 % in citric acid (pH 2).
This means that there are many soil conditions in which the phosphate is available for plants and crops. Pot tests have already confirmed that plants can utilize P from the Krepro phosphorus fraction. In certain growing environments Krepro's phosphate has proved to prevent nutrient leakage to the surrounding waterways, especially with plants of long growth period. (see picture at end of article)
How safe is sewage sludge?
Most urban areas in the western world are today connected to advanced sewage treatment with stringent effluent standards. The sludge from sewage works is, if spread on farmland, regulated by strict quality requirements set by the authorities; e.g., heavy metals and organic toxins must be low but despite considerable interest from farmers, mainly because of the valuable phosphorus content, the spreading of sludge on farmland is always under scrutiny.
For instance, in Sweden there have been some alarming reports lately dealing with contamination caused by silver, PCBs and brominated flame-retardants.
The Swedish food industry does not accept fresh food produced from farmland that has come into contact with sewage sludge, and the Farmers' Association has recommended their members to avoid sludge spreading on farmland, no matter how clean this sludge is.
While this is the current situation in Sweden, many other countries are likely to express the same concerns in the future. Disposal of organic waste will be prohibited or strictly limited after 2005-2006 in many countries.
The remaining options for sludge will then be controlled landfill, composting or incineration. All these options are expensive and volume reduction is important to reduce costs. Current sludge treatment processes today are designed to dilute the sludge or reduce its volume. Composting or mixing the sludge with lime normally produces only 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 utilizing the phosphorus contained in the sewage sludge.
Volume reduction of the dewatered sludge can decrease the cost for landfill and incineration. To decrease the water content is a costly. It is possible to use more polymers in the dewatering process, but this does not significantly reduce the water content
in the sludge cake. Sludge drying demands energy (12,000 MJ per ton of DS) and expensive investments.
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 hydrolyzed by thermal treatment followed by digestion the organic content can be reduced to 40% but also by this treatment the inorganic part remains as solids, and the contents of dangerous substances like heavy metals remain the same, but now in a higher concentration.
Separation technology
The KREPRO process is based on a completely new separation technology able to separate valuable products from municipal wastewater sludge. Four main products are being recovered from the sludge:
• Fiber fraction for energy production
• Phosphate fraction for use as a fertilizer
• Precipitant for recycling into sewage plants
• Carbon source for nitrogen removal
The process is a modular process which can be accommodated to local conditions and interests, with options to achieve one or several of the following targets: volume reduction (always); phosphate production; nitrogen removal; heavy metals removal; precipitate 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
• Fiber fraction separation
• Phosphorus precipitation
• Phosphate separation
• Recycling of precipitate and carbon source
Authors' Contacts: Juan Gonzalez, Kemira Iberica, Kemwater and Ingemar Karlsson, Kemira Kemi AB, Kemwater, email: [email protected], [email protected]
