The settling behaviour of activated sludge in wastewater treatment plants (WWTPs) is a common
operational test used to assess performance and monitor changes in the treatment process. In particular,
the sludge volume index (SVI) has long been recognised as a useful index of plant operation. Recent
surveys of plant managers and custodians at small and medium-scale activated sludge (WWTPs) in
Ireland indicate that, despite being a simple test and providing informative data, the SVI tests are
generally not utilised. Similar trends can be found in other countries, particularly, at small facilities
without resident staff. At all plants surveyed, a variation of volumetric settleable solids is the only
routine operational settlement test being carried out; test frequency varies from daily to weekly. Most
commonly, the test comprises a 30-min period of quiescent settling in an Imhoff cone (‘cone test’). The
resultant volume of sludge settled is the recorded, however, cone test results are not comparable to the
standard methods defined stirred settled sludge volume (SSV) test. Furthermore it cannot be used to
determine SVI and is thus of limited value. The settleable solids test, on which the cone test is based, is
recommended for dilute sludges, which are frequently not representative of activated sludge processes.
This study compared stirred and quiescent 30-min settlement tests carried out in 1.3L and 2Lsettleometer vessels, cone tests and quiescent settling in standard 1L-graduated cylinders; activated
sludge samples (n = 37) from 8 small-medium scale activated sludge plants were tested. The results
show that (i) 30-min quiescent settling in an Imhoff cone generally yields higher settled volumes than
quiescent settling in a 1.3L or 2L-settleometer vessel, (ii) equivalent results are derived from an Imhoff
cone and a graduated cylinder and (iii) quiescent settling in 1.3L and 2L-settleometers yield higher
results than stirred settling in the same vessels. A correlation is observed between the quiescent settling
in the 1.3L-settleometer and SSV determined as per Standard Methods. A relationship is also presented
to derive SSV from quiescent settling. This may facilitate estimation of SVI using simple apparatus. The
results presented indicate that by replacing the vessel used in the ‘cone test’ with a settleometer, more
meaningful information, that could significantly improve facility management, can be derived. Future
work is focused on establishing that dilution can significantly reduce settlement test-time while
maintaining SVI measurement accuracy.