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The KREPRO process for the treatment of sewage sludge is based on a completely

new separation technology combining old knowledge with a new creative way of

thinking. All 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 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

Cupper

Cadmium

Mercury

Cromium

Zinc

Nickel

Lead

PCB(polychlorinated

biphenols)

PAH(polyaromatic hydrocarbons)

Nonyl phenol

Toluene

20000

80

50

1500

23000

1200

2000

5

80

2000

170

100

<5

1

220

1000

150

180

<0.1

<3

<5

<5

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.

Separation technology

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.

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