Influence of pH on cuprosolvency in hard water.

Project leader: Erik Levlin

Project group:
Experimental test rig: Fredrik Göhle Vattenvårdsteknik, KTH.
Reference group at Uppsala Gatukontor: Rolf Bergström and Bernt Fredriksson

Project period: 1990-1993

Finansing/contractor: Uppsala Gatukontor

Experimental set up

Uppsala municipality have had problems with high copper content in the sewage sludge, originating from dissolution of copper from copper pipes. The dissolution of copper can be reduced by increasing the pH-level. However, sinceUppsala has a very hard water an incresed pH-level will couse problems due to precipitation of lime.

To study if the dissolution of copper from pipes can be reduced by incresed pH-level and water softening, dissolution of copper from copper pipes has been studied in a experimental test rig at Uppsala Gatukontor Water Laboratory in Uppsala. The test rig has 6 lines with copper pipes there 2 lines have untreated water and the other lines has been pH-adjusted by flow proportional dosage of 4 M NaOH to 2 different pH-levels. For each pH-level one line has been charged with 50 °C water which has cooled in the pipe and the other with cold water. The exposure time is determined by electric timers which with regular flushing give exposure times of 60 hours, 12 hours, 4 hours, 2 hours, 1 hour and 30 minuts. Analys of water samples was made at Uppsala Gatukontor och sweep electron microscope analyse of corrosion products was made by Inger Odnewall at Corrosion Science, KTH.

Test serie 1 — Minimal pH-increase

In test serie 1 the effect of a minimal pH-increase to pH 7.8 and pH 8 has been studied. If the pH-increase is smal the copper contents may be reduced without causing problems due to lime precipitation.

The pH-increase have given smaler copper content by precipitation of unsoluble corrosion products, malachite in the cold water pipes and cuprite in the hot water pipes. Howver, the reduction of the copper content was too smal and lime precipitation occured in spite of the smal pH-increase.

Copper content against pH for test serie 1 with minimal pH-increase.

Copper content against time for test serie 1 with minimal pH-increase.

Test serie 2 — Large pH-increase

In test serie 2 a larger pH-increase to pH 9 and pH 8.3 was made. An increase of the pH-level to 9 gave a smaller dissolution of copper. The calcium content was reduced to a quarter and the alkalinity to the halth. A larger decrease of alkalinity hade given further reduced copper content.

Examination of corrosion product showed signs to pitting in form of spoted green presipitation in the cold water pipes and points with copper oxide and copper chloride in the hot water pipes. This will probably not couse pitting. If the alkalinity had been further decreased the risk of pitting have increases.

Copper content against pH for test serie 2 with large pH-increase.

Test serie 3 — Desalination and pH-increase

In test serie 3 the water was desalinated before pH-adjusting to pH 9 and pH 8.3. The effect of pH-increase of softened water and the possibility to counteract lime precipitation by desalination was studied. Two third of the water flow was passing the ion exchanger. The alkalinity decreased from 270 to 180 mg HCO₃/litre which is one third. Other ion content decreased with 66%-74%.

Use of ion exchanged water caused that the variations in copper contents was larger. The highest values there at the same levels as earlier but many analysis with lower contents was achieved, giving a lower mean copper content. Desalination with a third was not enough to prevent lime precipitation if the pH-level is higher than 8.2. Problems with instability in pH-value coused by the ion exchance was noted, thereby it should have been better to control the NaOH dosage with an pH-electrode.

Copper content against pH for test serie 3 with desalination and pH-increase.

Reports

Levlin E. (1993) Inverkan av pH på kuprosolvens i hårt vatten. Försök utförda vid Uppsala gatukontor. (Influence of pH on cuprosolvensy in hard waters. Tests made at Uppsala gatukontor) TRITA-VAT 1931, Vattenvårdsteknik, KTH, 47 sidor.
Bilaga 1: Kopparhalt mot pH för försöksserie 1 med minimal pH-höjning.
Bilaga 2: Kopparhalt mot tid för försöksserie 1 med minimal pH-höjning.
Bilaga 3: I. Odnewall Analys av kopparprover.
Bilaga 4: Kopparhalt mot pH för försöksserie 2 med stor pH-höjning.
Bilaga 5: I. Odnewall Analys av kopparprover.
Bilaga 6: Kopparhalt mot pH för försöksserie 3 med avsaltning och pH-höjning.

Levlin E. (1991) Kuprosolvens i hårda vatten. (Cuprosolvensy in hard waters)
TRITA-VAT-4911, Vattenvårdsteknik, KTH.