Wastewater treatment challenges faced by the petrochemical and refinery industry, and opportunities for water reuse

Industrial wastewaters (WWs) from refineries and petrochemical and chemical sites have several common features with municipal wastewater in terms of organic pollution that can be removed in a final centralized, biological process.

Often the biodegradability of the ‘chemical oxygen demand (COD)’ in industrial WWs can be less than that of the municipal wastewater or the initial concentration higher. The discharge limits for the treated effluent are often the same as for municipal wastewater, leading to a need for higher removal efficiency, however.

As for municipal wastewater treatment systems, the biological processes need to be installed downstream of the primary treatment. The main difference is that, for municipal wastewater, primary treatment removes only inert materials and grit, which are common to all inflows, while, typical pre-treatments for industrial WWs are:

• - dedicated to specific streams that are then combined into the final, common, biological polishing unit;

• - based on various unitary processes specific to certain streams and their key pollutants – e.g. oil removal; metal precipitation; H₂S, ammonia or VOC stripping, etc. In general, industrial wastewater treatment requires a ‘system’, not a ‘plant’, consisting of multiple units dedicated to one or more influent streams.

• - designed for equalization, so that pollutant flows and mass loads are balanced, enabling the final, central, biological unit to operate as a process reactor, fed with known proportions and amounts of different streams, thus enabling the final quality of the treated water to be guaranteed. Equalization, like pre-treatment, must be provided as a system, incorporating multiple units dedicated to different wastewater streams.

One of the main challenges concerns industrial wastewater composition, which affects the process, construction material selection, and the potential for re-use, perhaps because of salinity or total dissolved solids (TDS). TDS, especially chloride, sulfate and sodium, if added in the industrial production process, are not removed by conventional treatment (only desalination is effective). Sometimes they can cause issues in a biological treatment plant:

• - adverse impact on biomass flocculation, perhaps leading to solid-liquid separation issues in the clarifier, especially if the salinity concentration changes frequently;

• - above certain TDS thresholds, slow nitrification kinetics and potential process instability arise, as well as impacts on sludge growth; TDS concentration can also cause issues for industrial processes discharging to surface waters where discharge quality is prescribed;

• - if the chloride concentration is high, great care must be taken in material selection (especially for aerobic processes), as well as in COD analysis and related biological plant performance evaluation, regardless of the analytical procedures used; and,

• - treated effluent containing high TDS concentrations is less appealing for re-use than a typical, treated, municipal effluent.

Other challenges with industrial WWs that affect treatment selection and operation are:

• - temperature, in areas where environmental and/or process conditions generate effluents with temperatures above those suitable for biological processes – e.g. nitrification;

• - some metals with complex chemical behavior are contained in typical effluents – e.g., selenium from refineries and in mine WWs;

• - certain streams may be irrelevant in terms of hydraulic flow but contain high concentrations of key parameters – e.g. spent caustic (characterized by high pH, TDS, COD, TKN, H₂S) – and often lead to high cost/effort in pre-treatment.

In other words, industrial wastewater streams, if not properly managed, holistically with full water cycle optimization, can lead to high CAPEX and OPEX solutions. These can be avoided if the treatment process selection approach is appropriate. Three examples are presented in this paper; the first two relate to the industrial area (petrochemical + refinery) of Venice Porto Marghera (Italy) and the third to the Refinery of Bahrain.

Porto Marghera is an industrial complex near the Adriatic Sea, roughly 15 km from Venice in North-Eastern Italy.

Industrial WWs from Porto Marghera are pre-treated at each production plant and collected at the central industrial WWTP, for physical, chemical and biological processing. Venice Lagoon, an extremely sensitive environment, receives the WWTP discharges. Special legislation has been used to try to safeguard the area, setting concentration limits for industrial and urban WW discharges. To meet these, it has been necessary to upgrade industrial WWTPs with new technologies.

The biological treatment system at Porto Marghera comprises two separate but identical lines, each including pre-denitrification, oxidation/nitrification and UF membrane filtration. To improve denitrification and comply with TN regulatory limits for discharge, the existing post-denitrification tanks, which can treat roughly half the total permeate flow rate, remained in operation.

Full details of the MBR project, start-up, and plant performance, are provided in Cattaneo et al. (2008), Cattaneo et al. (2011). Tables 1–3 provide the main parameters.

The WWTP's final effluent needs to comply with the Ministry of Environment's limits – Table 2,DM 23/4/98 (1988). Table 3 shows the regulatory limits for ten substances considered significant and important, and the removal efficiencies required DM 30/7/99 (1999).

With the upgrade complete, the WWTP's effluent was subjected to regular chemical analysis for 5 months from start-up. Samples were taken at the bioreactor's inlet and outlet, and after post-denitrification They were determined for Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), Suspended Solids (SS) Total Kjeldal Nitrogen (TKN), nitrate, nitrite, organic and total nitrogen, total phosphorus, micro-pollutants, heavy metals and organic solvents, as well as the ten substances listed in Table 3.

Analytical quantification of major macro-pollutants in the bioreactor's influent is reported in Table 4, with the same species in the treated permeate given in Table 5. The COD/BOD₅ ratio in the permeate increased to between 3.8 and 4.4 (average 4.19), confirming removal of biodegradable COD – the remaining COD was recalcitrant.

Cattaneo et al. (2008) analyze and discuss the micro-pollutants, the principal results, and required and achievable removal rates for the substances of concern, are given in Table 6.

With reference to micro-pollutants (metals, chlorinated organics, BTEX and the chemical species/groups of concern), the WWTP's multi-barrier treatment, including a first chemical step followed by biological and physical processes, promotes metal removal. In general, HM removal was between 15 and 50% in the chemical step, and in the following biological step between 10 and 98%, depending on the metal Masotti & Verlicchi (2006).

For chlorinated organics, BTEX and persistent pollutants – typically, persistent COD – the main removal mechanisms are: (i) chemical co-precipitation and absorption in sludge in chemical/physical treatment, (ii) absorption on biological sludge and biodegradation due to metabolic and co-metabolic reactions in the MBR, and (iii) suspended solids retention by ultrafiltration. Thus, the sequence of physical/chemical, biological and ultrafiltration treatments enhances micro-pollutant removal, as confirmed by Qin et al. (2007).

Some observations on petrochemical and similar WWTPs are valid here. To meet a 10 mg L⁻¹ limit for TN:

• - Full stable nitrification is required (this also meets a 2 mg L⁻¹ limit for NH₄-N);

• - A multi-stage plant (anoxic-aerated-anoxic-aerated), including post-denitrification, is necessary to ensure full control of oxidized forms of nitrogen;

• - Complete removal of TSS is mandatory. This enables, nitrogen compounds associated with the floc (8 to 10% of biomass, depending on solids retention time (SRT)) can be retained efficiently from (biological) secondary effluent.

Furthermore, even when the best available technology (BAT) consists of a 4-stage MBR, the TN limit of 10 mg L⁻¹ cannot be guaranteed if the influent's recalcitrant organic nitrogen content is high. This was not the case at Porto Marghera but, in some WWTPs, even 4-stage nitrogen removal plus an MBR may not be sufficient (ATV, 1995; Pehlivanoglu-Mantas & Sedlak 2006, 2008) if the influent's soluble organic nitrogen content is much greater than 10 mg L⁻¹. Wastewater-derived dissolved organic nitrogen poses considerable challenges to WWTP designers, as most physical and chemical treatment processes cannot remove low molecular weight, hydrophilic compounds. This is difficult with respect to regulation.

The Mestre-Fusina-Marghera area had two main treatment plants:

• - PIF, an integrated polishing and post-treatment platform for industrial and domestic WWs, and stormwater, and,

• - the Petrochemical site's industrial treatment facilities, with an MBR and incinerator.

Between 2003 and 2009, the evolution in environmental care and economy of the Venice area changed the volumes and compositions of the streams to be treated and managed. The volume of industrial wastewater decreased, while domestic wastewater and stormwater both increased. As a result, the industrial WWTP had spare capacity that needed to be used to reduce the specific WW treatment costs (€/m³) and achieve more sustainable operation, while the PIF plant was closer to hydraulic saturation and in need of further improvement to treat the expected volumes of domestic wastewater and to supply water for re-use.

The aim of the study was to identify the best and most sustainable integration option between the PIF and Petrochemical Site plants, from both environmental and economic aspects.

The best integration option between the PIF and SPM plants was identified to enable:

• - Reorganization of water and sludge streams to promote treatment specialization (i.e. industrial wastewater taken to the petrochemical MBR, and closure of plant sections ‘doubling’ the function of others, such as post-denitrification there);

• - Maximizing the use of treatment lines characterized by relatively constant operational costs (e.g. the petrochemical MBR);

• - Improvement of system water re-use, producing different qualities of water for re-use:

  • o  Keep the wetland effluent (at least 3,900 m³ h⁻¹) as cooling water.

  • o  Produce demi water (for use at the petrochemical site and industrial area) from WW, increasing proportional re-use with UF, RO, and mixed bed demineralization

The new demi water line could have received either the MBR effluent from SPM or the wetland influent; the wetland treated effluent is better because its TDS is lower.

Treating all industrial WW at the petrochemical site, and domestic and storm waters at PIF, allowed the specialization of both, with maximization of treatment efficiency and minimization of operating costs, because:

• - capacity saturation of the petrochemical MBR plant enabled minimization of operating costs and maximization of the volume treated, and allowed for best possible treatment for industrial WW, increasing the overall level of environmental protection;

Use of the treated biological effluent upstream of the wetland as a source for the new UF + RO process train, enabled the volume of water re-used in the system to be increased. The ratio of re-used to treated water will increase to 77%, from the 60% defined by the PIF project for the municipal wastewater;

The increase in re-use provides additional benefits, like enabling conservation of surface water for potable or other higher value uses, and decreasing the volume discharged to the sea, along with the corresponding energy costs for pumping.

A key success factor was defining the minimum required water quality for each purpose, from the beginning.

With the water quality targets defined, the treatment steps/systems could also be defined. In this context, the wetland was confirmed as viable and environmentally friendly, with minimal need for supervision, while UF + RO enabled production of demi water.

Bahrain Petroleum Company (BAPCO) operates the Bahrain Refinery, near Sitra, Bahrain. The refinery has primary treatment facilities for process wastewater. Oily wastewater is collected in the oily water sewer system from the process units, tank farms, and other areas, and transferred to a battery of American Petroleum Institute oil water separators for initial separation of free oil. The separator effluent is treated in an induced air floatation (IAF) unit, whose effluent was combined with other streams of essentially clean or relatively unpolluted water before being discharged to the sea. In 2006, BAPCO started testing the WW for secondary treatment and found that biological treatment would be necessary to meet the projected treatment requirements to be established by Bahrain's Supreme Council for the Environment (BSCE).

Although the biological treatment of refinery WWs is common, the BAPCO WW has some abnormal characteristics that, together with the discharge consent limits to applied to meet local conditions, make biological treatment a challenging task, including:

• - stringent discharge limits, especially for nitrogen (5 mg L⁻¹ TKN), due to the characteristics of the receiving body – The Gulf

• - high salinity (up to 30,000 mg L⁻¹ TDS, with 18,000 mg-Cl L⁻¹) and temperature (up to 48 °C in summer), which strongly affects bacterial activity and biological treatment performance

• - low concentration of degradable organic carbon (COD) compared to other refinery WWs, resulting in insufficient organic carbon for denitrification

Due to the low TSS and TKN discharge limits, and the wastewater's high TDS and temperature, MBR treatment was selected, which overcame the poor flocculation of CAS arising from high salinity. MBR has become more accepted recently, especially where discharge limits are stringent and/or water re-use is being considered. Until now, however, MBRs have not been used for refinery wastewater treatment, mainly due to the potential risk arising from WW oil content and the lack of track-record for similar, large, industrial applications.

A pilot test was used to characterize the wastewater influent streams and assess MBR performance. At the end of the pilot phase, treatment of spent caustic in the anoxic biological compartment was also investigated, to treat this WW using an economic and ‘green’ approach. The purpose was to assess whether spent caustic could be treated properly in the bioreactor, exploiting its COD and alkalinity to reduce supplemental chemical requirements while, simultaneously, disposing of the waste by a low-cost process. The test results enabled an optimized biological process design to be developed to meet the discharge limits. Marinetti et al. (2009) provide details of pilot plant operations. Tables 7 and 8 summarize the main characteristics of the IAF effluent and pilot plant operation.

Due to the relatively slow growth of autotrophic (nitrifying) bacteria in this wastewater matrix, the pilot plant was operated using a SRT of 35 to 45 days. After two months of operation, acetic acid was added to the anoxic zone of the plant to maximize nitrate removal (denitrification). After spent caustic addition, the acetic acid dose was progressively decreased to allow the biomass to acclimatize better to degrade the COD in the spent caustic. Although the TKN concentration of the spent caustic is high, the added load was negligible because of the low flow rate. The COD removal performance of the pilot plant was not affected by the addition of spent caustic and neither was nitrogen removal. Stable nitrification confirmed that the spent caustic had no negative or toxic effect on the biomass. Average spent caustic characteristics were: 68,571 (+/ − 8,918) COD; 21,767 (+/ − 2,472) sulfide, and 894 (+/ − 439) mg L⁻¹ TKN. Although the concentration of sulfide in the spent caustic was high and contributed approximately 21 mg L⁻¹, permeate concentrations were always far below the discharge limit. Spent caustic also provided alkalinity, decreasing the need for fresh caustic to be added. The average fresh caustic addition, with spent caustic, was only 30% of the previous requirement, without spent caustic CH2M HILL (2009).

The equalization, sludge de-watering, spent caustic management and chemical dosing systems were part of the WWTP, which also required the use of special materials due to the high chloride content. The plant design included a double stage cooling system, enabling WW temperature reduction from 48 to 35°C. The first stage – cooling from 48 to 38°C – is achieved with plate heat exchangers using cooling tower water as a cold source. The second stage uses another set of plate heat exchangers receiving cooling fluid from a lithium-bromide chiller.

The salinity of the Sitra sludge was around 2,500 mg-TDS L⁻¹, an order of magnitude lower than the refinery's wastewater. A sudden increase in salinity can disrupt biological cells due to osmotic effects, causing total loss of the biological population performing organic removal and nitrification.

After mid-November 2013, spent caustic was injected into the first anoxic cells of the biological trains. This started about fifteen days before the performance test, so that the biomass had been acclimatized by increasing the flow rate slowly over a period of time. The average spent caustic characteristics (different from the pilot plant period) were: 83,040 COD; 35,536 sulfide; 2,938 TKN and 1,240 phenols, all as mg L⁻¹.

The maximum load performance test of the WWTP started in late November and lasted thirty days. Around 910 m³ h⁻¹ of process wastewater were fed to the plant for treatment to meet the effluent discharge limits. Process data were collected daily to maintain WWTP performance, observing the required effluent water quality. A four-stage configuration consisting of an anoxic-aerobic-anoxic-aerobic treatment sequence was used. SRT was 35 to 40 days and sludge was recycled from the membrane tank to the first aerobic stage at a ratio of 3, while the nitrate recycle was from the first aerobic stage to the first anoxic stage at a ratio of 2. During the test, spent caustic was also fed to the first biological anoxic cells in line with the design data as shown in Table 9. The plant proved to be able to treat the maximum loads and meet the effluent discharge limits.

BAPCO WWTP is a new benchmark in industrial WW treatment and, received the’ Distinction award for Industrial Water Project of the Year’ at the Global Water Summit 2014 held in Paris (7-4-14). It is also an example of a high quality and sustainable approach.

In parallel with construction of the new MBR, the best options for permeate re-use were evaluated. Its characteristics make it suitable to replace or be added to supplies from existing water sources – e.g. seawater and brackish water from wells.

Re-use alternatives identified were the barometric condensers (sea water used currently), cooling tower make-up water, and the fire water system (brackish water, now). The study's priorities were to reduce brackish water use and the volume of water discharged to the sea, as far as possible, and increase the number of cycles of concentration of the cooling towers.

Further scenarios were developed, based on treatment unit installation on the recycle line (oxidation and NF membranes) to control COD, TKN and TDS concentrations, and increase re-use.

Refinery, chemical and petrochemical WWs may contain a great variety of macro- and micro- pollutants requiring differing treatments for removal. More stringent regulatory limits for discharges have led to the upgrading of many WWTPs, with design choices dictated by many different constraints.

Upgrading of the main industrial WWTP at the Porto Marghera petrochemical site, Northern Italy, and the new, MBR-based WWTP at BAPCO's Bahrain refinery are up to date plants able to achieve complete nitrification/denitrification and degradation, as well as bioaccumulation and mineralization of recalcitrant compounds, to meet strict effluent discharge limits. Such plants cannot deal with soluble refractory nitrogen, however. The BAPCO case is an example of how to deal with extreme operating conditions – e.g. high TDS and temperature – as well as managing specific toxic wastewater streams, like the spent caustic in Bahrain, using proper plant design, and appropriate selection of start-up and acclimatization procedures.

The most appropriate approach for industrial wastewater treatment involves not only the ‘end-of-pipe’ WWTP but holistic consideration to find the most sustainable way of integrating the WWTP into an existing wastewater management and re-use system. This can be seen in reorganization of the water cycle in the Venice area and the concept of water re- use at BAPCO.