Issue 02/2017

From Science and Research
Biopolymers from municipal
waste water treatment plants
Polyhydroxyalkanoates (PHA), which are biodegradable
polyesters accumulated by more than 300 different
microorganisms under nutrient limited conditions are
a source for bioplastic production [1, 2, 3]. Bacteria mostly
use PHAs as an intracellular storage compound for energy
and carbon [4]. The general chemical structure of PHA can
be seen in Figure 1. Depending on the length of the side
chain (R), PHAs are classified as short chain length (SCL) or
medium chain length (MCL) PHAs [5]. Most material characteristics
of SCL resembles polypropylene (PP) [6]. Therefore,
PHAs are most likely used as a substitute for PP [7].
PHA production on municipal waste water
treatment plants
Pittmann and Steinmetz [8, 9] were able to show the
possibilities of PHA production on waste water treatment plants
(WWTP) at different processing conditions. In a two-staged
production process, as shown in Figure 2, firstly short chained
volatile fatty acids (VFA) are produced. The PHA production itself
(Stage 2b) is based on a bacteria mixed culture selection from
excess sludge of a WWTP via aerobic dynamic feeding (Stage
2a). The installed feast/famine regime for enrichment of PHA
producing bacteria is state of the art and tested by many authors
[10, 11, 12]. PHA producing bacteria in the WWTP’s excess
sludge are able to use the polymers as a carbon- and energy
source during the period of starvation (famine phase) and thus
gain a selection advantage [13]. After a period of enrichment,
the biomass contains a high proportion of PHA accumulating
bacteria and is transferred to Reactor 2b for PHA production.
The whole production process takes place in a bypass to the
WWTP, and therefore without impacts on its cleaning capacity.
However, the usage of primary sludge for VFA production and
the further usage of these acids for PHA production removes
up to 39 % of the primary sludge’s chemical oxygen demand
(COD) [8]. Hence, the PHA production process competes with
the biogas production on the WWTP.
Results
At first different raw materials of a municipal WWTP regarding
VFA production were observed with primary sludge showing
the best mas flux results [8]. Through variations of the process
parameters temperature, pH, retention time (RT), withdrawal
(WD) and the mode of operation (batch or semi-continuously) of
the reactor, a maximum VFA mass flow of 1,913 with a VFA
concentration of 1,653 could be achieved [8].
Afterwards experiments regarding the PHA production
were conducted. Through variations of the process
parameters substrate concentration, temperature, pH and
cycle time, PHA concentrations up to 28.4 % of the cell dry
weight (CDW) could be achieved [9].
Potential analysis
On the basis of the named results and detailed data about
the amounts of sewage sludge on WWTPs a potential analysis
was calculated. The goal of this analysis was to determine the
potential for biopolymer production on German WWTPs. The
used input parameters for the calculations are shown in Table 1.
After calculating the amount of the primary sludge in Germany
and with respect to the fact that roughly 92 % of the people
equivalents (PE) are connected to WWTPs of the classes IV and V,
at which it can be assumed that these facilities produce primary
sludge, the theoretical reactor size can be calculated. Now, date
from the experiments can be used to calculate the possible
amount of biopolymers produced at German WWTPs. The PHA
production sums up to 157,000 [16]. Under consideration of
By:
Timo Pittmann
TBF + Partner AG,
Böblingen, Germany
Figure 2: Scheme for
production of PHA
including potential
calculations for
German WWTPs
20 bioplastics MAGAZINE [02/17] Vol. 12