Iwa international Specialist Conference

70.

Phosphorus recovery by one- or two-step technology with use of acids and bases

Kristina Stark* and Bengt Hultman

Royal Institute of Technology (KTH), Dep. Land and Water Resources Engineering,

S-100 44 Stockholm, Sweden

*[e-mail]
ABSTRACT

The interest of sludge fractionation for product recovery has increased due to difficulties to find suitable sludge disposal options. A more sustainable solution for large wastewater treatment plants would be to use an efficient combination of removal and recovery of constituents from wastewater, such as phosphorus. The method of using sludge fractionation is considered to be sustainable where the sludge is seen as the raw material from which products are recovered.

Sludge management has been under debate during the last years in Sweden due to e.g. difficulties to obtain consensus on agricultural sewage sludge re-use. The first suggested requirement of phosphorus recovery (75%) seems to be much lower than earlier proposed. The final decision from SEPA will be 15 Oct 2002.
In the Scandinavian countries phosphorus is mainly removed from wastewater by chemical precipitation. Phosphorus is then bound strongly to metal ions and for phosphorus release technologies as KREPRO and BioCon the technology chosen is combined thermal treatment and leaching of phosphorus by use of acids.
Studies have shown that technology systems with thermal treatment and dissolution of phosphates from the sludge by use of acids have about the same chemical demand for phosphorus recovery. The demand for chemicals is much dependent on the type of method for phosphorus removal (biological or chemical) and increases linearly with the dosage of precipitation agents.
A solution may be to use two-step technology i.e. recovery of phosphorus by two ways, one using biological technology and the rest of the phosphorus is recovered by an advanced technology system (see Figure 1). This technology may be suitable for a wastewater treatment plant if the phosphorus recovery demands are high (over 75%). Use of enhanced biological phosphorus removal and fractionation of the sludge in two stages is advantageous both with respect to low necessary chemical and energy demands and recovery efficiency. The wastewater treatment plant may choose one system or combined the two.

Phosphorus phosphorus Phosphorus rich

rich poor water

return sludge return sludge

Recycling of

phosphorus poor return sludge

Primary sludgem Excess sludge

& filter sludge

phosphorus rich

Digested Digested leachate

sludge sludge

Ammonium rich supernantant Sludge residual

Figure 1 Two step technology for recovery of phosphorus rich water streams
The paper will present one solution for the advanced technology system and compare the differences in recover phosphorus as two-step technology or only as one system, i.e. different recovery rates.
Studies have started to recover phosphorus by use of calcium oxide from sludge residual from supercritical water oxidation. The sludge residual is first leached by use of base. The experiments are conducted in room temperature. Complementary recovery experiments will be performed and the results will be evaluated and included in this paper.
Problems with phosphorus recovery systems need to take into account the presence of iron e.g. used as precipitation agent and aluminium e.g. from zeolites, clay, water works sludge and/or from precipitation agent. Iron- and aluminium ions can bind phosphate and thereby make it more difficult to obtain an effective phosphorus recovery. It should also be an effective separation of the organic and inorganic material to receive a phosphorus product with low part of organic material. It is also important with a separation of the toxic materials from the phosphorus product.
The main criticism towards use of processes as KREPRO and BioCon is the high chemical demand for phosphate release from sludge or ash and the following recovery of phosphates. This chemical demand will decrease much if partial biological phosphorous removal is introduced. Alkaline leaching of the sludge residual from the Aqua Critox (supercritical water oxidation) is another possibility to significantly reduce the chemical demand for phosphorus release followed by phosphate recovery as calcium phosphate.

71.

P-recovery from sewage for the elementary P-industry
Abraham Klapwijk & Hardy Temmink

Sub-dpartment of Environmental Technology, Wageningen UR
Abstract
In this paper the possibilities to recover phosphate from sewage for the elementary P–industry with a focus on the Netherlands are reviewed. Two scenarios are evaluated and the end products of these scenarios are compared to the quality that is required by Thermphos (a Dutch producer of elementary P). The first scenario is incineration of a mixture of primary and secondary sludge from wastewater treatment plants or from bio-p plants. The second scenario is P-extraction from secondary sludge produced in wastewater treatment plants with bio-P removal.
In table 1 the quality of ash from sludge from mixed chemical and biological P-removal plants, and sludge from bio-P plants only is compared to the requirements set by the elementary P-industry. In general we can conclude that ash from incinerated sludge does not provide a good source of secondary phosphates for the elementary P-industry.
Table 1 Quality ash compared with requirements Thermphos

	P2O5 (g/kg ash)	Zinc (mg/kg ash)	Iron (g/kg ash)
Ash sewage sludge (primary + secondary sludge) ^1)	190	3500	100
Ash bio-P sludge (secondary sludge) ^2)	360	3100	16
Requirements for reuse by the P industry	> 250	< 100	< 10

¹) based on STOWA (2001), ²)based on analysis of sludge from one bio-P plant
Another route to recover phosphate from sewage is to extract the phosphate before the sludge goes to the sludge treatment. We restrict to the feasibility of P-extraction of sludge before sludge treatment to bio-P plants. After the phosphate has been extracted from the sludge, and the phosphate-rich water is separated from the sludge, the phosphate has to be precipitated. In table 2 the quality of the precipitate of the Crystalactor® (Ca-precipitate) and of Al-precipitate are compared with the requirments set by the elementary P-industry. Based on the metal contents the quality fits the requirements of Thermphos .
Table 2 Quality precipitate compared with requirements Thermphos

	P2O5 (g/kg ash)	Copper (mg/kg ash)	Zinc (mg/kg ash)	Iron (g/kg ash)
Phostrip phosphate precipitate (Crystalactor®)^1)	260	1.6	35	-
Aluminum phosphate precipate ^2)	474	<30	<300	2
Requirements ThermPhos	> 250	< 500	< 1000	< 10

¹)STOWA (2001), ²) based on some experiments with bio-P sludge from the sewage treatment plant of Bennekom
72.

Phosphorus and coagulant recovery by alkaline sludge treatment
Kaoru KATO, SANKI ENGINEERING CO., LTD.

Research and Development Section, Project Planning Department, Environmental Systems Administration Division

1742-7, Shimotsuruma, Yamato-shi, Kanagawa 242-0001, Japan

Abstract; Recently, the effluent standard of nitrogen and phosphorus was reexamined for the prevention of the eutrophication in Suwa Lake river-basin sewerage since 1995. The simultaneous phosphorus precipitation process has been introduced as a phosphorus elimination countermeasure. The phosphorus is surely removed by the flocculant, and it is finally removed as sludge and incinerated ash. However, phosphorus is rare resource. It has estimated, if the amount of phosphorus which occupies 10 - 15% among an import phosphorus rock as a fertilizer is contained in the municipal wastewater. Therefore, it seems to be more important that it is recycled than disposing of the phosphorus in the sludge. Then, the establishment of the technology that recovered and recycles the phosphorus in excess sludge of the simultaneous phosphorus precipitation process was tried.

On the recovery method of phosphorus and flocculant, devised system flow is shown in Fig.1. PAC(Poly Aluminum Chloride) is used as a flocculant. In the water treatment process, the phosphorus is removed by the liquid with the function as a flocculant recovered from excess sludge. The insufficient PAC is supplied, and the phosphorus elimination is done perfect. By the alkali treatment of excess sludge, phosphorus and flocculant component, which are the immobilization thing, are eluted, and it is separated to concentrated sludge and supernatant liquor. The high-dense phosphorus in supernatant liquor becomes insoluble to the hydroxyapatite in calcium chloride. By appropriately adjusting pH of flocculant component, which remains after the phosphorus recovery in the supernatant liquor, it can be recycled as a flocculant for the phosphorus elimination.

The effectiveness of recovery method of phosphorus and flocculant was verified by the basic experiment. As a result of alkali treatment (pH12) of excess sludge of the simultaneous phosphorus precipitation process, the phosphorus which corresponded to 60% of phosphorus concentration 218mg/l in the sludge eluted it to the supernatant liquid. And, Al which corresponded to 86% of Al ( the flocculant component ) concentration 568mg/l in the sludge eluted it to the supernatant liquid. It was confirmed that phosphorus and Al eluted to the supernatant at the high proportion. In addition, excess sludge of Al content rate 9.3%-ds was treated with alkali, and got supernatant liquid was adjusted in [Ca]/[P] mole ratio of 1.7 with calcium chloride. As the result, the PO₄^3--P concentration of the supernatant lowered from 95mg/l to 28mg/l, and the phosphorus recovery rate became 70%. The Al concentration of the supernatant changed from 570mg/l to 520mg/l, and it remained for 91%. Using the high-dense supernatant liquid with Al, phosphorus elimination in the sewage was tried. By adjusting pH of the supernatant liquid under 3, it was possible to obtain the result of exceeding the phosphorus elimination ability by the PAC.

On the assumption of the sewage treatment plant of scale 100,000m³/d, mass balance calculation on P, Al, BOD and SS was tried. In the simultaneous phosphorus precipitation process, the consumption PAC quantity becomes 273kgAl/d, and in the recovery method of phosphorus and flocculant, it becomes about 1/3. In the simultaneous phosphorus precipitation process, SS generation rate ( after the centrifugal concentration ) becomes 5,723kg/d, and in recovery method of phosphorus and flocculant, it becomes a decrease of 49%. The phosphorus recovery quantity by the recovery method of phosphorus and flocculant becomes 223kgP/d, and it becomes the recovery of 59% for influent sewage 380kgP/d. On the basis of the mass balance calculation, chemical cost and sludge disposal cost were calculated. As the result, in the simultaneous phosphorus precipitation process, it was estimated with \ 21.0/m³-sewage, and in the recovery method of phosphorus and flocculant, it was estimated with \ 13.4/m³-sewage.

Since Suwa Lake river-basin sewerage is a hot-springs resort belt, its arsenic concentration is comparatively high. The action of toxic substances, such as arsenic in the phosphorus recovery thing, will be reported in applying this system.

Fig.1 System flow of the recovery method of phosphorus and flocculant

73.

abstract:

Current researches: P-recovery from ashes

Prof. Dr.-Ing. Peter Cornel, Dipl.- Ing. Christian Schaum,

Technische Universitaet Darmstadt (Germany)