Page 54 - FoodFocusThailand No.163 October 2019

SOURCE OF ENGINEER
SOURCE OF ENGINEER
Phosphorus recovery

and reuse from wastewater

Phosphorus (P) is essential to human life and vital for
food production. It is the critical building block of DNA,
cell membrane and bones and plays a crucial role in
cellular energy metabolism. Today, P is mostly obtained
from mined phosphate (Pi) rock, but natural reserves of
Pi rock are concentrated in a limited number of countries
such as Morocco, China, and the US. On the other hand,
an inefficient use of P and the leakage of phosphate-
containing fertilisers, detergents and sewage into water
bodies are causing irreversible eutrophication problems.
Moreover, mined Pi rock is largely contaminated with toxic
heavy metals such as cadmium and radioactive uranium.
From health and environmental perspectives, there are
increasing concerns about the long-term application of
chemical fertilizer to farmland.

Increasing attention has been paid to the development of P refinery
technology that can recover P from waste streams and reuse recovered
P products for agricultural and industrial purposes. In the wastewater
treatment sector, P is removed from wastewater using chemical or bio- P compounds for manufacturing high-value added products including
based technologies. Removed P ends up in sewage sludge which is semiconductor, lithium battery, liquid crystal panel, medicine, and fire-
then subjected to anaerobic digestion, dewatering and incineration. This retardant plastics. Elemental P, called white or yellow P, is the essential
offers hot spots for P recovery from (i) the rejected water from sludge starting material for the manufacture of high-purity P compounds. The
dewatering, (ii) digested sludge, (iii) and incinerated sludge ash. secured supply of elemental P is becoming increasingly difficult in the
Many full-scale P recovery plants are currently operating in Europe, global market. Actually, the supply risk is much higher than that of Pi rock.
North America, and East Asia. Basically, the P recovery technologies EU added elemental P to the list of its critical raw materials, taking into
are (i) chemical Pi leaching from incinerated sludge ash, (ii) Pi salts account the potential risk of the secured supply of high-purity P compounds
precipitation, and (iii) struvite crystallization after anaerobic sludge in Europe.
digestion. Incinerated sludge ash having the high content of P is also Elemental P is now produced by only four countries in the world,
used as a raw material for the manufacture of phosphoric acid in a wet including China, USA, Kazakhstan, and Vietnam. The production of
acid process. elemental P is an energy-intensive process which is strongly dependent
P recovery practices are now expanding not only to the wastewater on locally-sourced electricity, Pi rock, and cheap labor forces. It requires
treatment sector but also to the manufacturing sector. In the approximately 14 MWh of electricity for each ton of elemental P produced.
manufacturing sector, Pi must be removed from wastewater to meet Moreover, since Pi rock is contaminated with toxic heavy metals and
stringent effluent regulation in areas vulnerable to eutrophication. The radioactive elements, pollution control is another difficult problem regarding
recycling of recovered P products as a fertilizing material can save the the production of elemental P from Pi rock using a conventional arc process.
costs of sludge disposal and leads to the significant reduction of plant To solve these issues, it is necessary to redefine the P value chain
operating expenses. P recovery is also practiced from solid waste through technology and business innovation based on recycling. The
streams such as animal manure and steelmaking slag. In East Asian technology innovation needs to promote the development of (i) highly-
countries, including China, Korea and Japan, steelmaking slag is one efficient P recovery from secondary resources, (ii) an improved wet acid
of the most important secondary P resources. Recovering P from process to generate phosphoric acid from recovered P products, (iii) an
steelmaking slag allows the rest to be reused as raw materials in blast innovative carbothermal reduction of low-grade Pi to elemental P with
furnaces. This has the enormous potential to improve the resource minimum electricity consumption and low environmental burden, and (iv)
efficiency of steelmaking process. new processes for the manufacture of high-value added P compounds to
Except some European countries such as Switzerland and Germany, meet the demand from high-tech industries. Among them, the key
no regulation requires P recover and recycling for the wastewater technology is the innovative carbothermal reduction of low-grade Pi to
treatment sector. This allows the wastewater treatment sector to consider elemental P.
P recovery as an extra service. On the other hand, fertilizer companies The technology and business innovation based on P recycling, called
cannot accept recovered P products unless they bring some economic P Innovation, can make a great contribution not only to the sustainability
benefits to their business. P recycling practitioners need to establish of agriculture but also to the secured supply of high-purity P compounds
stable channels for the distribution and sale of recovered P products. to the high-tech industry. There is no significant tradeoff in the use of P
To make P recycling business more attractive, it is critical to develop a between agriculture and industry, because industry needs only a small
new value chain that can extract the maximum value from secondary P fraction of recovered P for their business. Rather, redefining the supply
resources. chain of high-purity P compounds can offer economic incentives to P
East Asian countries are becoming increasingly the center of high- recovery from secondary P resources, thereby making a great contribution
tech industries in the global economy. They need high-purity to the sustainable use of P not only in agriculture but also in industry.

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