Optimised design and equipment of secondary settling tanks enable a fundamental improvement of their performance in comparison to state-of-the-art technology.

For secondary settling tanks we are able to prove verifiably, that their hydraulic capacity can be significantly improved compared to the technology that is currently referred to as best available. This can be achieved by continuous minimisation of the internal loadings. Due to hydromechanical reasons these actual internal loadings are considerably higher than those loadings upon which dimensioning of the tanks is usually based. Applying hydrograv know-how this minimisation is technologically achieved by avoiding simultaneously hydraulically critical situations that arise during load changes dry weather-stormwater-dry weather and are simultaneously critical in terms of quantity and quality. That is how in operation the effluent quality can be raised to come close to the optimum compared to the current state-of-the-art.

Background:
The two secondary settling tanks, shown in figures 1 and 2, are charged exactly with the same dry weather loading (red sludge volume loading, blue surface loading)
q_SV = 290 l/(m² h),
q_a = 0,7 m/h).

Complying with current design directives both tanks are dimensioned exactly equally. But it is noticeable, that the tank in figure 1 contains much more sludge than the second tank, whose inlet design was adjusted to the hydraulic optimum on the basis of hydrograv know-how. Instead of 0,95 m clear water height (blue area) over the sludge bed (green to red area) at dry weather the optimised tank with app. 1,91 m sludge storage volume has almost 50 % more free volume available.


Figure 1: Secondary settling tank according to current best available technology with unfavourable inlet design: high internal loadings

Reason: taking fundamental physics into consideration in the first tank there is installed a "virtual pump" that stirs up the sludge having already settled. This hydraulic phenomenon is called "entrainment" in hydromechanics. The chart below the pictured tank demonstrates this: instead of continuous settling of sludge combined with reduction of internal loadings (which is the basic assumption in current design directives), the latter increases dramatically near the inlet. Due to entrainment ("virtual hydraulic jet pump") as much sludge is stirred up in the tank of figure 1, that the actual tank loading becomes more than three times higher as the dimensioning load at the same dry weather:
(q_SV)_int = 1040 l/(m² h)
(q_a)_int = 2,3 m/h

Problem: According to state-of-the-art technology nowadays almost in every secondary settling tank there is installed such a "virtual hydraulic jet pump" with high power.

Solution: figure 2 shows how the internal loadings change after the inlet is optimised for a reduction of this internal pump effect.

Initially the internal loading rises here as well (entrainment is a natural phenomenon that cannot be avoided, however, minimised). But the increase is considerably attenuated because of the hydrograv-optimisations (same dry weather situation):
(q_SV)_int = 465 l/(m² h)
(q_a)_int = 1,3 m/h

At the same external loading the actual loading is now much lower than in the non-optimised tank; thus, because of the optimisation for dry weather, the clarifier is by far not that heavily charged. By continuous and gentle adaptation of the inlet tank volumes can be reduced considerably. This applies for either construction or rehabilitation of secondary settling tanks. Discontinuous changes in loading create impulse surges on the density layered fluid body. These, in turn, generate density waves that negatively affect the effluent quality. Therefore hydrograv-solutions emphasize especially the minimisation of discontinuities and the attenuation of impulse surges.

Figure. 2: Secondary settling tank, improved performance with hydrograv-approaches: drastically reduced internal loadings enable an enhanced external LOADING

Copyright Notice:
The concept of actual loadings is developed in the doctoral thesis of Dr.-Ing. Martin Armbruster, managing director of hydrograv GmbH. In that concept tank-internal conditions are suitably converted and then related to currently used dimensioning parameters and derived values - e.g. sludge volume loading or surface loading (each with differentiation and comparison of internal and external values). Therefore this concept, approaches derived from this concept, the thought model of "virtual internal pumps" or "internal hydraulic jet pumps", respectively, and related models describing internal entrainment processes in reactors used in waste water treatment is the intellectual property of Dr.-Ing. Martin Armbruster and hydrograv GmbH.

Utilisation for purely scientific purposes is, of course, permitted if it is undertaken with a reference to hydrograv (www.hydrograv.com) and to Dr.-Ing. Martin Armbruster as the originator.

Every other way to utilize this concept, derived concepts and all related terms and definitions, also if changed, the use on other websites, duplication, storage, processing, and reproduction in databases or other electronic media or systems, as well as reports, brochures, and every other kind of publication requires preliminarily a written agreement of hydrograv GmbH. We explicitly object to every kind of commercial utilisation by third parties.

From a hydraulic point of view it is necessary to optimise internal loadings for improving the performance of secondary settling tanks.
However, we, as experts for hydraulics, explicitly point out, that this is not solely sufficient to guarantee a secure operation at a considerably higher hydraulic loading. The publication of further necessary measures to achieve this is in preparation.