Issue 1 [March]

Biological treatment has been carried out in two different systems: aerated closed and threephase
fluidized bed reactors for hydrocarbons removal from refinery wastewaters. For the two
systems, hydrodynamic study allowed the determination of operating conditions before
treatment experiments. Then, in a second time, biological treatments have been conducted in
the same operating conditions. The obtained results showed that in the three-phase fluidized
bed we can degrade hydrocarbons more rapidly than in a closed aerated bioreactor.
Among the different appropriate techniques available to create efficient contacts between
phases, the three-phase fluidization G/L/S where carrier particles are moving inside the
reactor seems very interesting. It allows an intimate contact between phases and present
many advantages concerning hydrodynamic and mass transfer phenomena. In fact,
depending on operating conditions and the bubble flow behaviour, the three-phase fluidized
bed could display different flow regimes
In these systems called bioreactors the solid particles covered with a biofilm are fluidized by
two ascending flows of air and contaminated water. With favourable operating conditions,
from a hydrodynamic and mass transfer point of view, the pollutant can be biologically
degraded up to 90%.
Until this date, the three-phase bioreactors modelling remains very complex because it
required taking into account several factors: the pollutant biodegradation rate in the biofilm,
the bioreactor hydrodynamic characteristics, and the reactant interfacial gas-liquid and liquidsolid
mass transfer. Thus the essential purpose of modelling is to integrate the microbial
kinetics with the reactor hydrodynamics. We can notice that a few models have incorporated
both bioreactor hydrodynamics and microbial kinetics.
For the steady state bioreactor model, we generally assume that the particles are uniform in
size, the biofilm is uniform in thickness, and the biofilm can be considered as homogeneous
matrix through which oxygen and substrate diffuse and are consumed by the microbes. The
liquid phase in the bioreactor substrate is considered to be axially dispersed while the gas
phase is assumed to be in plug flow [2]. Rittmann (1997) proposed a model based on wake
theory for predicting bed expansion and phase hold-ups for three-phase fluidized bed
bioreactors. In this model he modified the correlation for the computation of the bioparticles
drag coefficient CD [3]. He also attempted to explain the biofilm detachment which can occur
with three broad patterns: erosion, sloughing and scouring and assumed that the factors
affecting detachment rates can be grouped into two categories (physical forces and microorganisms
physiology in the biofilm).

Asymmetric multilayer Al2O3 ceramic membranes with pore sizes ranging from 3 to 500 nm
are synthesized in tubular form with external diameter of 14mm, internal diameter of 8mm and
length of 340mm. The membrane synthesis took place on commercially available supports,
with the dip-coating technique either from aloumina particle suspensions or from boehmite
sols.
The membranes are subsequently mounted in a pilot-scale module able to accept six
specimens, and used in micro- and ultra- filtration experiments for the purification of aqueous
streams from suspended solids. The experimental module is equipped with a back-flushing
circuit that can be activated on demand to prevent the fouling of the membranes and the
associated reduction in permeability.
For the microfiltration experiments membranes with 100nm pore size are used. In the case of
an aqueous solid suspension with a concentration of 0.1 to 1 wt.% consisting of solid particles
with an average size of 0.5 μm, complete solid rejection is observed. The process has a
capability of treating 0.8 m3 of feed per hour per square meter of membrane surface under an
average pressure difference of 3 x 105 Nt m-2. The fouling of the membranes can be quite
effectively reduced by back flushing at regular time intervals. Under complete retentate
recycling conditions, more than 95% of the feed volume can be recovered as microparticle
free water.
For the ultrafiltration experiments membranes with 3 nm pore sizes are used. In the case of
an aqueous solid suspension of nanoparticles with a concentration of 0.1 to 1 wt.% consisting
of solid particles with sizes of 20-30 nm the rejection was also almost complete. The process
has in this case a capability of treating 0.16 m3 of feed per hour per square meter of
membrane surface under an average pressure difference of 3 x 105 Nt m-2. Fouling appears
not to cause serious permeability drop in this case probably because even after the
nanoparticle deposition the membrane hydraulic resistance is the permeability determining
step. Almost the entire feed volume can be recovered as nanoparticle free water under
complete retentate recycling conditions.
Purification experiments are also performed in olive oil mill wastewater. Best results are
achieved by using a two step membrane process with gradually decreasing pore size.
Although complete rejection of solids and significant reduction of the BOD5 and phenol
content of the wastes is achieved, the very low permeability is the main draw back of the
process.

This paper presents a biological method for controlling odor problems caused by H2S
originating from sewer networks under anaerobic conditions. The proposed method is based
on the continuous addition of nitrate which oxidizes dissolved sulphide according to an
autotrophic biological procedure and inhibits further sulphide production by sulphate reducing
bacteria, until complete denitrification.
The proposed method was first tested at laboratory in a 3 l anaerobic batch reactor simulating
municipal wastewater of the city of Corfu in respect to sulphate concentration. Addition of
nitrate in non-septic (not sulphide containing) wastewater inhibits the production of sulphide
until complete denitrification. Heterotrophic denitrification rate was found 4.5 and 3.9 mg NO3-
N l-1 h-1 at 250C and 300C respectively. Higher C/N ratio is, probably, responsible for the
increased denitrification rate of the lower temperature.
Interestingly, addition of nitrate in a septic wastewater led to a preferential autotrophic
denitrification with sulphide as electron donor at a rate of 0.8 and 1.5 mg NO3-N l-1 h-1 at 25
and 300C. After complete sulphide oxidation, heterotrophic denitrification takes place inhibiting
any further sulfate reducing activity.
The effectiveness of the method was validated by field experiments in a 6.7 Km combined
sewer network in the city of Corfu, with an average wastewater flow of 500 m3 h-1, an average
retention time of 2 h and sulphide concentrations varying from 3 to 27 mg S2- l-1. Continuous
addition of 6.9, 15 and 27.7 Kg NH4NO3 h-1 for a period of 4 to 8 hours led to an average
sulphide removal efficiency of 84%, 98% and 99%, respectively.
Based on these experimental results, a continuous addition of 10 Kg NH4NO3 h-1 is proposed
for practical implementation as the optimal dosing, considering sufficient odor control and
tolerable increase of the ammonia load. The proposed method is not only effective but also
financially interesting taking into account the facility cost and the monthly operational cost,
during the summer months of the year.

This paper aims at a comparative evaluation of the total cost of urban sewage processing
units for several treatment systems, appropriate mainly for small capacity plants. The studied
systems are: Oxidation Ditch (O.D.), Trickling Filter (T.F.), Rotating Biological Contactor
(R.B.C.), Compact Sequential Batch Reactor (S.B.R.), Waste Stabilization Ponds (S.P.) and
Subsurface Constructed Wetland Systems (C.W).
For each system, operation and construction costs are calculated as a unit capacity function
for a 40-year operating period. Cost calculation is based on the analysis of its several
components such as energy consumption, chemicals consumption, personnel salaries,
maintenance expenses, construction materials and their respective quantities, required
mechanical equipment and land value. All pecuniary flows, in order to be comparable are
converted into current value.
Cost estimation is based, mainly, on the calculation of the several cost elements’ quantities,
with the development of standardized plans. Total and partial costs are expressed as
functions of plant capacity. The annual operation and maintenance (O&M) cost, the project
cost, as well as the total construction, maintenance and operation cost for 40 years of
operation (Present Value) can be written as a function of flow rate, in the form of a+bQ+cQ2;
whereas energy cost can be described by a linear function of flow rate, in the form of a+bQ,
with excellent approximation. The coefficients a, b, c, for the various treatment methods
examined and for every cost category are summarized in tables.
All treatment methods exhibited positive scale economies for the studied region of cost per
E.P.; this trend was especially evident for plant capacities up to 5,000 E.P.
Natural wastewater treatment methods present the least expensive option for the examined
range of E.P. The classification of the treatment methods from the least to the most expensive
one depends on plant capacity. The classification of the treatment methods based on their
cost for different plant capacities is provided.
The results provide a first level of information, which could be utilized for a quick financial
evaluation of the aforementioned systems as well as in the general decision making plan.