A mobile batch unit was used to test P-recovery by sorption onto CSH and fertiliser effectiveness was tested.

Amorphous calcium silicate hydrates (A-CSH) were synthesised as a slurry from sand and calcium hydroxide (lime). The A-CSH were tested in five batch runs in a 1000 litre mobile pilot reactor to recover phosphorus by sorption from sewage sludge digester dewatering liquor. The recovered P-A-CSH product was tested as fertiliser in pot trials on Komatsuna.

The production of A-CSH from sand and lime and laboratory P-recovery experiments are presented in SCOPE Newsletter n° 93. The A-CSH is a low-cost, easily produced material which takes up phosphate by adsorption, so that the P-enriched A-CSH (after P-recovery) can potentially be used directly as a fertiliser.

In this paper, a 1000 litre mobile pilot reactor (1.3 m high, 1.2 m diameter) installed on a small van was tested for five batch runs at a sewage works near Osaka, Japan. The sewage works uses biological phosphorus removal and the A-CSH reactor was fed with dewatering liquor from the anaerobic sludge digestate centrifuges. Because of flocs in the centrifuge liquor, it was necessary to pass this through a cotton filter before entry into the A-CSH reactor.

The reactor was stirred (2 blade agitator) and operated in batches: 1000 litres of liquor introduced and 590 – 720 g dry weight A-CSH was added, mixed for 20 – 60 minutes then settled for 30 – 60 minutes. 830 litres of supernatant was then withdrawn from the upper part of the reactor, and 170 litres of liquor/A-CSH mixture from the conical reactor base. The latter was filtered for 90 minutes in cloth bags to separate the A-CSH – recovered phosphate, as a cake with 83 – 87% water content.

The digestate dewatering liquor soluble phosphorus concentration was c. 175 mgP-PO4/l and 72-85% was removed by the A-CSH reactor. The recovered P-A-CSH product contained around 10% P dry weight.

Fertiliser tests
The recovered P-A-CSH product showed low heavy metal levels. It was tested (after drying at 105°C for 24 hours) for fertiliser effectiveness in 22 day pot trials using Komatsuna (Brassica rapa), soil pH 5.0, and gave better results than both calcium superphosphate and Gifu-no-daichi commercial fertilisers for leaf length (at 7, 14 and 22 days) and for live weight (22 days). Also, the P-A-CSH showed to not inhibit seed germination.

The authors conclude that amorphous calcium silicate hydrates show potential as a cheap and accessible sorbent material for P-recovery from sewage liquors, and that the recovered P-enriched A-CSH can be directly used as an effective fertiliser. The 1000 litre mobile plant provides a useful tool for testing and process development.

Concrete sludge
In a further paper, the authors tested acid-treated concrete sludge for phosphorus recovery from pure chemical solutions and real anaerobic sludge digester liquor, in beaker – 3 litre laboratory experiments. The concrete sludge is a waste from construction sites where more cement is generated than required. Sludge was collected from a ready-mix concrete plant, washed, dried, filtered to produce a cake, ground, then soaked in 1.3 M hydrochloric acid for 60 minutes. 70 – 96% phosphorus removal was achieved from synthetic and real wastewater after 5 – 60 minutes. Heavy metals in the P-enriched concrete sludge product were low and P content was 8.2% (dry weight), with the phosphorus content < 0.01% water soluble. The fertiliser value of the recovered product and its agronomic impacts remain to be demonstrated.

References
“Amorphous calcium silicate hydrates and their possible mechanism for recovering phosphate from wastewater”, K. Okano et al., Separation and Purification Technology, vol. 144, 2015, pages 63–69 http://dx.doi.org/10.1016/j.seppur.2015.01.043

“A mobile pilot-scale plant for in situ demonstration of phosphorus recovery from wastewater using amorphous calcium silicate hydrates”, Separation and Purification Technology, vol. 170, 2016, pages 116–121 http://dx.doi.org/10.1016/j.seppur.2016.06.040

“A simple technology for phosphorus recovery using acid-treated concrete sludge”, Separation and Purification Technology 165 (2016) 173–178 http://dx.doi.org/10.1016/j.seppur.2016.03.054

K. Okano, S. Miyamaru, Y. Yamamoto, K. Honda, H. Ohtake, Dept. Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan. M. Kunisada, Mikuni Pharmaceutical Industrial Co., Ltd., 2-35 Kamisu-cho, Toyonaka, Osaka 561-0823, Japan. H. Takano, Research & Development Center, Taiheiyo Cement Co., 2-4-2 Osaku, Sakura, Chiba 285-8655, Japan. M. Toda, Research & Development Laboratory, Onoda Chemical Industry Co., Ltd., 39-13 Miyamoto-cho, Itabashi-ku, Tokyo 174-0054, Japan.  

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