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The Basics of Chemical Phosphorus Elimination with Iron and Aluminum


Introduction

The impetus to eliminate phosphorus from sewage came from the heavily polluted lake of Zurich in Switzerland in the 1950s. The lake had suffered from algae growth and an anaerobic deep-water zone since about 1940. More than once it was infested with burgundy algae, the first time in the 1920s.

The lake of Zurich was created by the Linth glacier, about 10,000 years ago and forms, together with some neighboring smaller sister-lakes, a typical morainic landscape at the foothills of the Alps. Today on its shores and alongside of its tributaries lives a population of about 330,000 amidst some farm country. Back in 1950, the population was only 185,000.

Fig 1: Here we see the crescent shaped Lake Zurich. Männedorf, the village on the far shore was the place where worlds first sewage treatment plant with chemical phosphorus elimination went into operation in 1955. In the background you see the Alps.

The lake of Zurich is about 38 kilometers long and its width varies generally between 3 and 4 kilometers. Its maximum depth is 143 meters.

The lake serves as a major source of potable water, not only to the city of Zurich, but also to many villages alongside the lake. Apart of its recreational value, there still is some commercial navigation on it. Mostly gravel and stones are shipped.

In the early 1950s, first sewage treatment plants (STP) were built at the lake, with little effect to the water quality. It was the merit of Prof. E. A. Thomas (1912-1986), head of the Hydrobiological Department of the University of Zurich, to prove that phosphorus was the sole limiting factor to algae growth in the lake. Before 1950, it was disputed whether it was potassium, nitrogen or phosphorus or all three of them together.

To prove his theory, Thomas filled large transparent plastic bags with clear spring water to which he added the three fertilizers phosphorus, nitrogen and potassium in different concentrations and combinations and let them float on the lake. Thus he found that the only limiting factor to algae growth was phosphorus, since potassium and nitrogen were plentiful in most lakes on natural reasons.

For improveing the quality of the lake water, Thomas invented a simple process for eliminating phosphorus in sewage treatment plants: He added iron chloride-solution, and, as an alternative aluminum sulfate solution, directly into the activated sludge basin of the sewage treatment plant Männedorf in 1955 (Swiss Patent 361 543).
Today, this so called "Simultaneous Phosphorus Elimination" process is widely applied on sewage treatment all over the world as a standard method. Most engineers today don’t know that Thomas was the inventor. (By the way, Thomas never took fees from users of his patented process).

Fig. 2:
Prof. Eugen A. Thomas in 1983, aged 71 years
Inventor of the simultaneous phosporus precipitation in 1955


Theory:

Phosphorus is precipitated by useing ironphosphate or aluminumphosphate according to the following equations

With iron: Fe + PO4 ---> FePO4
With aluminum: Al + PO4 ---> AlPO4

The precipitation with aluminum is slightly better than with iron. See Fig. 2.

According to the mass-action law the remaining disolved phosphorus is inversely proportional to the molar surplus of metal salts. So the solubility (Sol) may be computed as:

(whereby K is the solubility constant
and "Me" stands for "metal", - iron or aluminum)
Sol = K • [mol Me]
[mol P]
= mg P/l


That means: By doubling the metal concentration the remaining phosphorus concentration is always cut by halve as demonstrated in Fig. 2.

Fig. 2: Precipitation of P from a NaH2PO4-solution useing Iron chloride or Aluminum sulfate at pH 7.5
Among Swiss sewage engineers, the mol-rate is called β-value.


Practical Application:

The role of the suspended solids on the final effluent of the plant

The phosphorus concentration of a final effluent of sewage treatment plants firstly depends on the molar surplus of the added metal-salt as demonstrated in Fig. 2. But also is dependant of the concentration of the suspended solids.
Usually the phosphorus concentration of suspended solids (SS) averages 3.5% total phosphorus with regard on dry matter. Typically, a secondary clarifier discharges an effluent of 10 mg/l SS (In USA the legal limit)
Maximum is 20 mg/l SS (legal limit in Switzerland)

Regarding the solubility curve in Fig. 2, it can be demonstrated that the phosphorus concentration of the effluent of a sewage treatment plant predominantly is governed the suspended solids, - far more then by the molar surplus of added chemicals.

Fig. 3:
Influence of the suspended solids on the concentration of total phosphorus of the effluent of a sewage treatment plant


This graph explains why it's impossible to comply with the Swiss law, when a plant's effluent has 20 mg/l SS (Suspended solids). Hence, a sewage treatment plant has to provide at least 15 mg/l SS or less!


An expected phosphorus concentration may be calculated with the following equation:

mg P/l =
0.4
β
.+ (SS•0.035)


How Phosphorus bonds to the digested sludge

In the early 1960's the simultaneous chemical phosphorus elimination theory came under heavy attack by kritics. Karl Wuhrmann, then professor for waste water biology at the 'SwissFederal Institute of Technology' in Zurich, joked about Thomas and stated that at digesting biological surplus sludge, phosphorus will be released by the omnipresent hydrogensulfide there, returning to the plant again. (Look publication Thomas: "Phosphat-Elimination in der Belebtschlammanlage von Männedorf und Phosphat-Fixation in See- und Klärschlamm" (Vierteljahresschrift der Naturforschenden Gesellschaft in Zurich, pages 419-434 (1965)), where Thomas contradicted Wurmann. I was a student of Prof. Karl Wuhrmann then, so I'm reporting firsthand.

In the eyes of Wuhrmann that would mean the once precipitated phosphorus is returning to the activated sludge process together with the supernatant from the digester, requesting an endlessly increasing demand of precipitants, Iron or Aluminum . This is an argument that sounds convincing to a chemical engineer because:

FePO4 + 2 H2S --> FeS2 + H3PO4 + H+

H2S (hydrogen sulfide-gas) is plentiful and omnipresent in any digester. It is a by-product of the decomposition of proteins. The solubility of ironsulfide is worse than that of ironphosphate what shifts the above equation towards the ironsulfide-side of the equation above. Here we see the phosphorus released indeed.

As a consequence of Wuhrmanns argument, Thomas made digesting tests in his laboratory with sewage-sludge and also with lake-sediments. He could prove, that the phosphorus was not released on digesting. With this finding everyone went back to normal and no one ever investigated why phosphorus miraculously remained fixed to the digested sludge. (When something is working well, why investigate why it does so?)

In the 1980s, - twenty years later, I once was involved in a study for precipitating ammonium simultaneously with the digesting process. By adding magnesia and phosphoric acid to fresh sludge of a sewage treatment plant, ammonia (NH4) is precipitated as

NH4 + H3PO4 + MgO + 5 H2O ---> Mg NH4 PO4 • 6H2O + H2O + H+
.........................................................................MAP


MAP is the short for 'magnesium ammonium phosphate'. It forms characteristic triangular cristals, flat plates, mostly about 0.2 to 0.5 millimeter per crystal. Through my weeks long study I became familiar with this type of crystal.

To my surprise, years later accitentaly I found the same crystals were abundant in any digester. That explained why Thomas was right and Wuhrmann surprisingly erred. Magnesium and ammonium are plentifull in all digesters, so the released phosphorus binds as MAP and won't be released at digesting as Wuhrmann claimed. Secret solved!

Fig. 4:
Typical MAP crystals in the digested sludge of the sewage treatment plant Tobl, S. Lorenzen (Italy). Crystal size: 0.2 mm.
Photo courtesy of Dr. K. Engl, general manager of the Tobl plant.

Last update: Feb. 2011

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