Why Using Polyaluminium Coagulants in Water Treatment

Advantages of polyaluminium coagulants include the following. low levels of residual aluminium in the treated water can be achieved, typically 0.01-0.05 mg/L. PACl and ACH work extremely well at low raw water temperatures. Flocs formed from alum at low temperatures settle very slowly, whereas flocs formed from polyaluminium coagulants tend to settle equally well at low and at normal water temperatures. Less sludge is produced compared to alum at an equivalent dose. Lower doses are required to give equivalent results to alum. For example, a dose of 12 mg/L PACl was required for treatment of a coloured, low turbidity water compared to similar performance obtained when using an alum dose of 55 mg/L, and the increase in chloride in the treated water is much lower than the sulphate increase from alum, resulting in lower overall increases in the TDS of the treated water.

Comparison of Polyaluminium and Alum

Polyaluminium coagulants are typically twice the price of liquid alum on per kilogram aluminium basis. However, lower doses of the coagulant and lower pre- and post-treatment alkali doses can still make its use economical. Polyaluminium chloride solution (10% Al2O3) is stable for 4 to 5 months when stored at less than 50oC and is so ideal for bulk storage and dosing installations. One possible disadvantage in using ACH/PACl relates to the removal of dissolved organic carbon (DOC) from water. It is well documented that effective DOC removal is possible with alum, particularly when coagulating at lower pH values using so-called “enhanced coagulation”. Alum appears to be a superior coagulant as far as removal of humic and fulvic colour constituents are concerned. A higher coagulation pH is adopted with polyaluminium coagulants and it possible that removal of THM percussors may not be as complete as with alum.

Advantages Introduction

The advantages of using PACl in regard to the effect on treated water TDS and sulphate levels are apparent. Note also a small improvement in the CCPP value of the treated water when using PACl. The chemical doses predicted from WaterQual when using PACl compare very well with actual requirements at the Water Filtration Plant. Estimated chemical costs using alum amount to $38.1/ML, compared to $41.3/ML with PACl. For an annual treated water production of 1200 ML, this translates to a saving $3900 per annum in favour of alum. The operational advantages of using PACl, particularly in cold months, make it an attractive coagulant. The THM level in the treated water is generally 40-50 mg/L when treating raw water with a true colour of 90-100 Pt/Co units. This is well below the current AWDG recommendation of 250 µg/L. In this instance DOC removal appears to be satisfactory.

The coagulant was found to be very effective and since changing over to PACl one of the most noticeable advantages noted has been the increased “robustness” of the water treatment process. Previously when using alum, plant performance was adversely affected after heavy rain. PACI has shown an ability to much better deal with these changes. The chemical’s ability to coagulate over a wider pH range is of enormous benefit in this instance. Another major benefit of using PACI has been a considerable reduction in the volumes of sludge trucked off-site to disposal. This has had a major impact on plant operating costs. Pre-treatment alkalinity adjustment using caustic soda has been greatly reduced since changing over to PACl. When a similar treated water quality is achieved in both instances with the CCPP higher in the case of PACl. An estimated $24/ML (or 14%) saving in total chemical costs is possible using PACl.