Development of the Phosphorus Recovery System (PRS) Utilizing Ultrasonic Wave in Incinerated Sewage Sludge Ash (ISSA)

*Ye Duk Choi, Jun Yeon Lee, Zoo Ho Jang, Jung Gone Joung, Kyu Mun Han, Mok Young Lee and Kweon Jung*

#### **Abstract**

This study was performed to develop a Phosphorus Recovery System(PRS) for the recovery of phosphorus from incinerated sewage sludge ash using struvite precipitation. Fly ash generated at the Seonam Sewage Treatment Plant(SSTP) has a high P2O5 content (13.9%). We developed a PRS consisting of an ultrasonic extractor, solidliquid separator, mixing tank, and phosphorus recovery tank. The ultrasonic extractor had a 28 kHz vibrator for high speed and efficiency, which could perform the extraction in one-quarter of the time required in the conventional stirring method. Results of tests on the ultrasonic extractor showed that up to 0.044 g of P per gram of ash could be extracted with 1 N NaOH at an L/S ratio of 10 mL/g and an ultrasonic output of 500 Wh for 0.5 hr. The PRS is needed to improve the operation method and economic analysis to commercialize the technology and its application through further studies.

**Keywords:** phosphorus, recovery, ash, sludge, ultrasonic

#### **1. Introduction**

Incineration ash generated from sewage treatment facilities is known to contain a large amount of phosphorus, about 10% [1, 2]. Republic of Korea relies entirely on imports of phosphorus, which is referred to as the 3rd element of fertilizer, in the form of phosphate rock. Phosphate rock is expected to run out of resources within the next 50 to 100 years because of the increase in world population and demand for fertilizers [1, 3, 4].

Recently, due to the prohibition of dumping of sewage sludge at sea, the rate of onshore disposal is greatly increasing. Phosphorus recovery from incineration ash has many advantages in terms of improving economic efficiency and minimizing volume of t reaction tank through high concentration of phosphorus concentrated in incineration ash. Most of the conventional phosphorus recovery technologies eluted phosphorus from sewage sludge under anaerobic and high-temperature conditions, but commercialization of the technology was not easy due to high water content and

low phosphorus content. Water content of sludge in incineration facility is more than 80%, and percentage of organic matter is higher than 60%.

Looking at the previous research results, phosphorus recovery is mostly in the form of MAP (Struvite, MgNH4 PO4 6H2O or MgNH4 PO4 H2O) and HAP (Hydroxyapaptite, Ca5(PO4)3OH) [5]. The principle of precipitation or crystallization is applied. Struvite can be expected to have a fertilizer effect because it contains essential elements necessary for plant growth, and excessive input is possible because it does not dissolve instantly due to the slow-release rate of nitrogen and phosphorus [6]. On the other hand, Struvite decomposes when heated and has high solubility in acidic conditions.
