An innovative aerobic bioreactor, the Biomass Concentrator Reactor (BCR), initially developed for resilient pollutants (e.g. MTBE) removal from groundwaters was tested to remove organic contaminants from tanker ships' rinse waters collected at the dedicated treatment facility in an Italian Port. The size and reduced operational requirements of this process unit would make it an ideal means of disposing such contaminated matrix directly onboard a vessel. The results obtained in this study are encouraging both for the quality of the obtained effluents and for the limited operational costs and complexity.

INTRODUCTION

Tanker ships' rinse water discharges constitute a serious environmental risk at global scale. Tankers that are designed for the transportation of crude oil must be in accordance with the regulations of IMO – MARPOL (IMO 1973); cleaning the tanks is one among the most important operations, on ships designed to transport liquid cargo, be it crude oil, petroleum products, or various chemical products. Although significant progresses have been made in this area (e.g. the development of ‘Crude Oil Washing’ (COW), based on using crude oil, i.e. the cargo itself, as the washing medium to remove residues from the tank surface by jets of crude oil, that are subsequently pumped out with the rest of the cargo) (Stojan et al. 2011), the use of sea water as a rinsing agent is still diffused in black- and white-oil carriers, with the result that large amounts of oily water must be properly disposed of. Treatment can occur at port in dedicated facilities (wastewater treatment plants with pretreatment) or at sea with different process technologies (Tsolaki & Diamadopoulos 2009). Oil tankers greater than 150 gross tons are required to have an oil-water separator (OWS) device on board.

An innovative aerobic bioreactor, called the Biomass Concentrator Reactor (BCR), initially developed for resilient pollutants (e.g. MTBE) removal from groundwaters and successfully demonstrated in laboratory and pilot scale applications (Venosa et al. 2006) was tested for this purpose. The initial design of the BCR was improved under a cooperative research and development agreement (CRADA) signed by the US EPA and UN.E.CO. srl, an academic spinoff of the University of Pavia (Italy), leading to re-design of the process in a more efficient, compact configuration (Figure 1), and allowing better performance and further reduction of volume requirements, making the process more competitive with mainstream conventional membrane bioreactors (MBRs). Although the underlying principle is very similar, that is, achieving high biomass concentration within the treating volume, mechanically separating it from the effluent, these must use pressure or vacuum pumps to produce the transmembrane pressure to allow water permeation, while the BCR relies on gravity flux alone to operate, under very limited head (up to max. 5 cm of H2O). In addition to reduce operational energy requirements, the limited pressure on the BCR's membranes also limits filter fouling and allows a longer operational duration of the filtration medium, and the low sludge growth rate results in low waste sludge volumes to be disposed.
Figure 1

(a) a full-scale BCR and b) interior view.

Figure 1

(a) a full-scale BCR and b) interior view.

The BCR was tested in the USA and Italy for the removal of MTBE and other gasoline –originated compounds in groundwater (Capodaglio et al. 2010) with full respect of applicable legal limitations. In this study the BCR was tested to remove organic contaminants from tanker ships' rinse waters collected at the dedicated treatment facility in an Italian Port. The size and reduced operational requirements of this process unit would in fact make it an ideal means of disposing such contaminated matrix directly onboard a vessel. The results obtained in this study are encouraging both for the quality of the obtained effluents and for the limited operational costs and complexity.

MATERIALS AND METHODS

Tank rinse water samples were collected at the dedicated facility in the Italian Port of Genua and homogeneized. Collected water samples were then decanted prior to treatment to separate oil content (Table 1, Figure 2). After verifying the chemical analysis of samples, in order to render the water more amenable to biological treatment, some of the oil recovered was added to the treated liquid, in order to achieve an appropriate BOD5/COD ratio.
Table 1

Waste water characteristics

    Decanted water Undecanted water 
PARAMETER UoM   
pH unità pH 
TOTAL SS mg/L 50 2,000 
ASH 0,11 0,11 
COD mgO2/L 191 1,120 
COD filtered mgO2/L 158 237 
BOD5 mgO2/L 55 800 
Chlorides mgCl/L 567 650 
Chlorates mg/L 1,9 1,9 
Free Chlorine mg/L <0,03 <0,03 
HC < 12 mg/L 7,3 1,968 
HC > 12 mg/L 4,7 1,339 
Fe mg/L 8,42 8,77 
Mn mg/L 0,6 0,47 
Ni mg/L <0,057 <0,057 
Cr mg/L <0,054 <0,054 
Pb mg/L <0,027 <0,027 
Zn mg/L 0,13 0,3 
    Decanted water Undecanted water 
PARAMETER UoM   
pH unità pH 
TOTAL SS mg/L 50 2,000 
ASH 0,11 0,11 
COD mgO2/L 191 1,120 
COD filtered mgO2/L 158 237 
BOD5 mgO2/L 55 800 
Chlorides mgCl/L 567 650 
Chlorates mg/L 1,9 1,9 
Free Chlorine mg/L <0,03 <0,03 
HC < 12 mg/L 7,3 1,968 
HC > 12 mg/L 4,7 1,339 
Fe mg/L 8,42 8,77 
Mn mg/L 0,6 0,47 
Ni mg/L <0,057 <0,057 
Cr mg/L <0,054 <0,054 
Pb mg/L <0,027 <0,027 
Zn mg/L 0,13 0,3 
Figure 2

Waste water samples.

Figure 2

Waste water samples.

A bench scale version of the BCR apparatus (4 L volume) was used for the tests, with the same type of filter medium used in the full scale version (1.2 mc). Nutrients were added by means of a membrane pump (Figure 3). HRT in the vessel was adjusted in order to follow the response and acclimation of the biomass to the waste: during the 36 days of experiments it was therefore reduced to up to half the initial value (from 4 to 2 days). Oxygen was supplied by air pumps through a bottom diffuser, with an average supply of 0.25 Nm3/h (3.8 L/min).
Figure 3

(a) the bench scale BCR; (b) a ‘naked'BCR’; (c) nutrients and air supply.

Figure 3

(a) the bench scale BCR; (b) a ‘naked'BCR’; (c) nutrients and air supply.

After three days of treatment at 20 °C (including biomass adjustment) abatement of the pollutant load in the effluent water was always greater than 85% (Figure 4). Biomass concentration in the reactor varied between 5 and 11 gVSS/L during the test period (Figure 5).
Figure 4

BCR Influent and effluent samples.

Figure 4

BCR Influent and effluent samples.

Figure 5

BCR biomass.

Figure 5

BCR biomass.

Figure 6

COD and BOD5 in the influent and effluent from the BCR.

Figure 6

COD and BOD5 in the influent and effluent from the BCR.

Figure 7

Total Susp. Solids in the influent and effluent from the BCR.

Figure 7

Total Susp. Solids in the influent and effluent from the BCR.

RESULTS AND DISCUSSION

The results of 36 days of operation of the bench scale BCR are summarized in the following Tables and Figures. COD and BOD5 concentrations in the effluent are independent of incoming loads and removal efficiencies close to 100% for BOD5 and greater than 85% for COD (Figure 6). It should be noted that no specific precautions were taken for the treatment of the samples, other than the addition of nutrients and micronutrients to the feed according to an empirical formulation developed in prior studies (Capodaglio et al. 2010).

The graph for SS shows abatement in the BCR always greater than 95% (Figure 7). This is due to the retaining properties of the filter medium (20 μm pore size). No fouling of the filter medium was observed during the test period.

Figure 8 shows hydrocarbon concentrations (classified as Total, light [C < 12] and heavy [C > 12]) in the influent and effluent. In previous experiences (Venosa et al. 2006; Capodaglio et al. 2010) the BCR process had shown excellent capacity of biologically degrading hydrocarbons.
Figure 8

Total, light (C < 12) and heavy (C > 12) hydrocarbons in influent and effluent from the BCR.

Figure 8

Total, light (C < 12) and heavy (C > 12) hydrocarbons in influent and effluent from the BCR.

In order to verify that hydrocarbons were actually degraded and not stripped by the air supply bubbling in the reactor, off-gases from the reactor were monitored towards the end of the test: results are shown in Table 2. Hydrocarbons in the off-gas are relatively low in concentration (the emission limit in the EU is 10 mg/Nm3) and decrease in time, as an indication that real degradation is occurring in the reactor.

Table 2

Off gas analysis from the BCR

date HC as C6H6 (mg/Nm3
30 August 4,7 
31 August 1,2 
03 September 1,8 
05 September 1,1 
date HC as C6H6 (mg/Nm3
30 August 4,7 
31 August 1,2 
03 September 1,8 
05 September 1,1 

Finally, the BCR's effluent toxicity was also evaluated with two protocols (respectively, V. Fischerii and Daphna Magna): inhibition at 30’ was on the average below 30% (between 35 and 85% in the influent) for the first, and zero for the latter (except for one single measurement of 10%, with influent varying between 0 and 100%), versus a legal requirement of less than 50% for discharge in surface waters (Figure 9).
Figure 9

Toxicity tests with (a) V. Fischerii and (b) Daphna Magna.

Figure 9

Toxicity tests with (a) V. Fischerii and (b) Daphna Magna.

The treated water thus complies with applicable regulations for discharge into public sewers (as industrial pretreatment) and in surface waters (as final treatment).

CONCLUSIONS

Tanker ships' rinse water discharges constitute a serious environmental risk at global scale, even though some progress is being made in handling this type of operations. Contaminated rinse water treatment can occur at port in dedicated facilities or at sea with different process technologies.

The BCR, originally designed for the removal of recalcitrant substances from groundwater, can successfully achieve removal of contaminants in this type of waters thanks to a high viable biomass concentration in the reactor (observed up to 11 gVSS/L during these tests), a low biomass growth rate, effective effluent filtration on the inner filter medium, and low head loss through the filter itself (low energy impact). Treatment of decanted water (to which some of the separated oil was added to provide substrate for the biomass) was carried out for a test period of 36 days and achieved at all times an efficiency greater than 85%, allowing discharge into surface waters according to EU regulations. BCR effluent was also tested for toxicity, obtaining largely passing results. Due to low biomass growth rate, very little amounts of waste sludge are generated and must be disposed of.

Treatment presented no specific problems or criticalities, as long as nutrients and micronutrients were added in appropriate quantities to the reactor's feed. Other requirements consisted in the supply of a constant stream of air to the bottom diffuser and in maintaining an HRT in the unit of approximately 2 days.

Based on these preliminary results, and on the ease of operation, the BCR could be adopted as part of a treatment scheme for this type of waste waters.

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