Site visits: Gifu alkaline sludge ash leaching, Kubota sludge furnace. Anders Nättorp, FHNW and ESPP Board, reports on a study visit to Japan, organised by the Phosphorus Recycling Promotion Council of Japan (PRPCJ) to discuss P-REX results and ESPP activities, with visits to two sites implementing recovery technologies unique to Japan. With thanks to Prof. Hisao Ohtake, Waseda Univeryity and Fumiki Hosho, KUBOTA Corporation, for help in preparing this summary.

The Japanese Ministry of Land, Infrastructure Transport and Tourism (MLITT) is conscious that the phosphorus in wastewater is a resource, but until now recycling was not a priority. Consequently phosphorus in sludge is only used for a small part through some application of sludge to non-agricultural lands. Mostly the sludge is incinerated and the ash is used for cement production or other construction purposes. Part of the sludge ash is also landfilled. Recycling is implemented full scale by three sewage plants that perform struvite precipitation and by two plants which apply the alkaline ash leaching process presented below.

PRPCJ members were curious to learn more about European experiences such as all the technologies available as shown by the P-REX project (www.p-rex.eu), the circular economy package and the opening up of the fertilizer regulation for recovered materials (See summary ESPP-meeting June 2016) and the resulting developments on the ground, for example the new production line for DCP from ash by Ecophos in Dunkerque or the FP7 Recophos process being piloted by ICL for future implementation (Link Ecophos Dunkerque ; ICL see SCOPE Newsletter n°120). The very fruitful discussions with industry representatives and university professors revolved around the reasons for the relatively positive political development in Europe compared to Japan; fertilizer quality and market; phosphorus and P4 supply security; the differences in wastewater treatment and sewage sludge disposal between Europe and Japan.  


Alkaline leaching of sewage sludge ash in Gifu, Japan

The Gifu city has four wastewater plants which treat sewage from 284 000 inhabitants. The sludge from these plants is incinerated in two sludge mono-incineration plants, generating around 710 tonnes of ash per year. In the past, the municipality used the ash to produce bricks for use in public construction works. Decreasing demand for bricks and high landfill costs for ash led to the search for alternative solutions.

The development of a new process for treatment of ash by alkaline leaching started in 2003 with funding from MLITT and collaboration between private sector (Metawater Co. Ltd, http://www.metawater.co.jp/eng) and the public sector (municipality of Gifu).

The process consists in leaching of the sludge ash by dilute NaOH (for 5-30 minutes at 50-70 °C), filtering, precipitation by Ca(OH)2 (for 6 hours at 20-50°C) and separation of the product by sedimentation and centrifugation, followed by drying to 20% DM. The precipitation liquor is recirculated.

The initial phosphorus content of the ash (30-35% P2O5) is lowered by around 30% (to approx. 22% P2O5), that is around 30% of P is recovered. Heavy metals leaching out of the ash into the phosphate product is lower with the alkaline leaching than with acid leaching, and is limited by keeping the P-recovery rate down to around 30%.

The leached ash is washed twice with water and then with an acid poly-ferric sulfate solution to immobilize the remaining metals. The leached ash now weighs slightly more than the initial ash weight before treatment (c. 750 tonnes/year wet weight), and its humidity has now increased to 30% compared to <0.5% originally. The leached ash is used as construction material or as soil amendment for tree planting (it still contains significant phosphorus).

Around 230 t/y of calcium phosphate (calcium hydroxyapatite) is produced (containing 27 t P/y). This has 29% citric acid phosphate solubility. It is well below the limits in the Japanese fertilizer control act for heavy metal limits (by factors between 3 and 100). Around 90% of the product is sold to the fertilizer industry and the remainder is granulated to standard size and sold directly for use as fertiliser to JA ZEN-NO (National Federation of Agricultural Cooperative Associations https://www.zennoh.or.jp/about/english/index.html) at 20 000 JPY/t, i.e. around 70% of the price of conventional fertilizer.

After successful lab experiments piloting and product registration, the full scale ash leaching plant was brought into operation in Gifu in 2010. The overall ash treatment cost is lower than the previous brick production and represents less than 3% percent of the total wastewater treatment cost.

Another slightly smaller full scale plant using the same alkaline ash leaching process is operated in the municipality of Tottori.

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Part of the phosphorus recovered in the Gifu plant is granulated and sold directly as fertiliser by JA ZEN-NO.


Kubota sludge furnace process

The Kubota Corporation with over 33 000 employees provides piping, agricultural machines, construction machines and equipment also delivers melting furnace technology. More than 30 furnaces have been constructed the last 40 years in Japan. Most of them are still in operation for the treatment of sewage sludge, ash, industrial and hazardous waste and gasification of pyrolysis residues. We visited one of the municipalities using Kubota furnaces for sewage sludge disposal since several decades. Kubota services operates the furnace, a common setup for smaller municipalities.

Dewatered sewage sludge is dried to 80% dry matter in order to have a self-sustaining incineration. In the furnace, dried sewage sludge is pushed from the circumference of the circular reactor towards its centre. As it approaches the centre, the material gradually increases in temperature and decreases in volume. It burns at the surface, forming a melt that continuously flows through a contraction into a lower chamber where the flue gas is diverted to the side and the melted material falls freely into a water bath. The burning zone has a temperature of about 1300°C and the lower chamber is above 900°C to avoid dioxin formation. The flue gases are led through boilers which recover energy for sludge drying and then through exhaust treatment. The plant capacity depends on the diameter of the oven, which can range from 1.2 m to 10 m (equivalent to 35 000 t dry matter per year).

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Schematic Diagram of Kubota Melting Furnace

Up to now slag has been used in the construction sector, for public works backfilling. The last years Kubota has developed the process to enable phosphate recovery. Through addition of iron oxide (Fe2O3) and careful control of the oxygen concentration the phosphorus can be kept in the melt whereas the more volatile heavy metals (Pb, Zn, Cd, Cu etc.) are removed in gaseous form. Nickel, part of the copper and some other metals remain in the phosphoric slag as metallic droplets. A further mechanical separation of these droplets is considered by Kubota as feasible.

In order to increase the plant availability of phosphorus in the recovered slag, calcium hydroxide is added and the melt is rapidly cooled with water. Thus a slag with >90% citric acid solubility of phosphorus and a P-recovery rate of over 90% can be achieved. Pot tests on rice at soil pH 5.5 show a fertilizer efficiency of 97% relative to single super phosphate.

The energy balance is also being improved by recovery of heat from the oven itself. Through this measure it is projected to be able to deliver an oven processing dewatered sludge without additional fuel (today 20l/t of fuel oil sludge treated).

It is estimated that sludge disposal costs in the melting oven are approximately 20% higher than mono-incineration. However, the value of the phosphorus in the slag is estimated to be equivalent to that in phosphate rock (based on rock prices for import into Japan), more than enough to make up the higher cost. Kubota is currently looking for interested partners and potential clients to implement this phosphorus recovery process.

Report by Anders Nättorp, FHNW – University of Applied Sciences and Arts Northwest Switzerland.          

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