(460f) Recovery and Reuse of Aluminum in Municipal Wastewater

Coagulant recovery is an attractive option from both economic and
environmental sustainability standpoints and has elicited the interest of
researchers over the past 10 – 20 years. However, these studies
focussed on recovery with minimal investigations of the recycled organics and
nutrients.  Therefore, in this work, method of
recovery of alum coagulant from primary sludge originated from treated raw
wastewater and its reuse potential have been investigated and assessed. The
sludge was generated using alum as a coagulant in a standard jar test (100 RPM
for 1 minute and 30 RPM for 20 minutes) from primary influent obtained from
Pottersburg Water Pollution Control Plant. This sludge
was later acidified with either HCl or H₂SO₄ to a pH of
0.5 – 2.5 to recover the aluminum (60 minutes
of mixing at 170 RPM to solubilize the
coagulant followed by 10 minutes of
centrifugation
 at
3700 RPM
for separation of the coagulant).  Efficiencies
increased with pH, varying from 82% - 90% (pH 0.5) to 70% - 78% (pH 1.5) to 39%
- 60% (pH 2.5). There was no significant difference in the results using either
HCl or H₂SO_4. In addition to that,
acid treatment reduced the sludge volume by 80% which would alleviate the cost
of sludge disposal. The cost of acid at $0.28/kg and taking into account the
incremental aluminum recovery at pH 0.5 vs. 1.5, operating at a pH of 0.5
rather than a pH of 1.5 would not be justified. Given the marginal improvement
in going from pH 1.5 to 0.5, on a cost performance basis, the optimum pH for
aluminum recovery was chosen as 1.5 and all further experiments were conducted
at this pH. The recovered coagulant was recycled
twice for coagulation of fresh batches of primary influent and the water
quality parameters (total suspended solids, phosphorous, nitrogen, and chemical
oxygen demand) were determined. For the fresh alum, the removal efficiencies of
total suspended solids, chemical oxygen demand, total phosphorous, and total
nitrogen were 85%, 65%, 80% and 33%, respectively. For the recycled aluminum, removals
of total suspended solids, chemical oxygen demand and total nitrogen were less
affected with coagulant recycle, with removals decreasing by approximately 10
percentage points in each recycle. This
efficiency implies that the recovered aluminum’s coagulation power is
effective, thus reducing oxygen demand and biological sludge volumes. Phosphorous was the most sensitive parameter during
coagulant recycling, due to the carryover of o-phosphate with the recovered
coagulant and increased by 1 mg/L and 3 mg/L in the
first and second cycles respectively. To mitigate the increasing
phosphorous load, struvite precipitation was
attempted by the addition of magnesium chloride and ammonium hydroxide in a
molar ratio of 1:1:1. This proved to be ineffective as it precipitated the aluminum.

Graphical abstract