Iwa international Specialist Conference

Introduction

Primary objective of waste water treatment is the purification of waste water to a quality that is acceptable for the receiving water body.

Sustainability requires in addition:

• 
  to withdraw pollutants such as heavy metals, organic halogen compounds, pharmaceutical or endocrine substances from the natural cycle and deposit properly
  
• 
  to recover resources from the waste water, e. g. the nutrients nitrogen, phosphorus, potassium, magnesium and calcium.
  

Every substance which is eliminated from the water phase during different waste water treatment steps remain either in the sludge or is transformed into the gas phase (e.g. CO₂, Methane, N₂). Thus the sludge contains most of the removed pollutants and nutrients.

The most important nutrient seems to be phosphorus, because it is an essential element for each organism and on the other hand because the resources are limited. It is estimated that the phosphorus sources of today’s quality meets the requirements mainly for food-production only for the next 60 to 130 years [Steen, 1998].

One way to use the nutrients of the sludge is the direct use as fertilizer in agriculture. But as in many other European countries it is discussed controversially in Germany because of it’s ambivalence containing nutrients and pollutants. It can be expected, that the use of sewage sludge in agriculture might be more and more restricted because of its potential harm. Only sludge with “very low” heavy metal concentration might be accepted [BVEL und BMU, 2002].

In 1998 the waste water treatment plants in Germany produced approx. 2.5 Mio. Mg DS sludge whereof 68 % was used in agriculture, 9 % came to a landfill and 18 % was incinerated. From 2005 on it won’t be allowed to deposit sludge with more than 5% organics at landfill.

So it is expected that large amounts of sewage sludge will be incinerated in future.

The Phosphorus balance at the waste water treatment plant (WWTP)

The phosphorus load into municipal wastewater treatment plants has been decreasing continuously mainly because of the use of phosphate free detergents. At present the phosphorus load amounts to about 1.6 to 2.0 g P/(PE · d) in Germany [Ries and Dai, 2001]. This is comparable with an phosphorus influent concentration of 8 to 10 mg/l.

The allowed phosphorus effluent concentration for most plants amounts to 1 – 2 mg/l by legal regulations. With 200 l/(PE  d) the specific effluent load will be 0.2 g  P/(PE · d). Thus about 90 % of the phosphorus must be eliminated in the WWTP.

Figure 1 shows a phosphorus balance for a typical waste water treatment plant in Germany. Approximate 11% of the phosphorus is separated by the primary sludge and 28% by the secondary sludge without any specific phosphorus removal. Thus approximately 50% of the phosphorus has to be eliminated additionally by means of Bio-P-processes, precipitation, or other phosphorus-removal-techniques. With the generally applied Bio-P and/or precipitation techniques 90 % of the incoming phosphorus ends up in the sewage sludge.

Thus it seems to be worth to develop methods for the extraction of phosphorus from sludge, or taking into account the future development towards sludge incineration, extracting the phosphorus from the ash.

F
igure 1: Example for a phosphorus balance at the waste water treatment plant in Germany
(PS: Primary sludge; SS: Secondary sludge; PE: Population Equivalent)

Phosphorus recovery from ash

At present the section wastewater technology of the “Institut WAR” at Darmstadt University of Technology investigates in cooperation with the Ruhr river water association (“Ruhrverband”) the recovery of phosphorus from ash. The two years project is funded by the environmental agency of the federal state of North Rhine-Westphalia.

Pre-trials show correlations between the phosphorus elimination process at the WWTP and the quality of the ash. The way of phosphorus elimination has an important effect to the chemical bondage of the phosphorus.

Similar experiences were found in Japan by Matsuo (1996) who performed leaching tests with ashes from enhanced biological phosphorus removal plants. The trials showed that that phosphorus was extracted by (hot) water only..

A correlation between the quality of the ash and the phosphorus elimination process at the WWTP were also found and described by Wiebusch (1999).

An additional aspect of our research project with “Ruhrverband” is to compare the ashes received from full scale and laboratory incinerators. The important question is how the incineration process influences the chemical bondage of the phosphorus and thus the extractability of phosphorus.

The presentation will present first results of the research project, i.e. leaching of phosphorus and heavy metals from different sludges (form plants with chemical-physical and enhanced biological phosphorus removal) and different incineration processes.

74.

Paula Ylinen and Reetta Puska 26.9.2002

Kemira Agro Oy

P.O. Box 44

FIN-02271 Espoo

Finland

RECYCLED PHOSPHATES AS RAW MATERIALS IN FERTILISER MANUFACTURING.

Natural phosphate minerals, such as fluoro- and hydroxoapatites are the main phosphorus sources for the fertiliser industry to day. Apatite rock is the raw material for phosphoric acid production. Both phosphoric acid as well as apatite rock are processed further to produce NPK compound fertilisers. We in Kemira Agro have studied whether these natural phosphate minerals could at least partly be substituted by recycled phosphates coming from waste management processes. Our experiences concern iron phosphates, recovered from Kemira´s sewage sludge treatment process Krepro.

When evaluating the suitability of the new raw materials for the phosphate industry, the total phosphorus (P) content of the material and its availability for the plants are the key issues. A very important criterion is also the ratio of impurities to the total phosphorus content. Some impurity elements may cause problems in fertilizer manufacturing, others in fertilizer use. As an example, for safety reasons, no organic contaminants are accepted in materials used to produce nitrate-containing fertilisers. Additionally, the new material must be free from pathogens and other toxic elements. Fertilizer industry is part of the human food chain and therefore all the parts of the chain must accept the grade and quality of the new raw materials used in the fertiliser production.
Logistics management is an important factor when selecting raw materials for fertilizer industry. Industry demands sustainable logistics with no smell or dust problems.

Recovered phosphates, such as calcium phosphate, iron phosphate and struvite have all been proposed as possible P-sources for agricultural soils. For the fertiliser industry, however, all these phosphates represent raw materials to produce efficient fertilisers with various NPK-compositions for many different applications. Compatibility with other fertilizer components during the manufacturing process and further in the farmer´s field is essential and needs to be studied.

Krepros iron phosphate has been utilized to produce forest fertilisers with phosphorus and potassium as the main nutrients. According to our results, iron phosphate is able to supply P for trees that grow on acidic peatland soil. The best growth results have been obtained with fertilisers with mixed phosphorus source, containing both apatite-P and FePO₄-P in their formulas. Additionally, in mineral-poor conditions iron of the fertiliser efficiently prevents soluble phosphates from leaching into the surrounding waterways. Presence of FePO₄ in the fertilizer thus ensures that there will be long-term P available for sustainable forest growth for many years.