Aiming at the problems such as the water quality is not up to the standard for serious emulsification and high stability of the site wastewater containing polymer and low efficiency of the equipment, as well as water clarifiers increase the aggregation of oil sludge causing sludge treatment flowsheet blockage, the optimization of the process of wastewater treatment is studied. The best combined process and operating parameters are obtained by applying electrochemical degradation technology, simulating the site unit equipment and developing process optimization [10] - [14] . And a set of 10 m³/d skid mounted wastewater containing polymer treatment plant was designed and manufactured, which was used for the field test of electrochemical treatment of wastewater containing polymer in CEPK platform of SZ36-1 oilfield.

This plant consists of the unit equipments of cyclone dosing mixing tank [15] , electrochemical oil-remover, inclined tube separator, walnut shell filter and so on, forming the whole treatment process of wastewater containing polymer including cyclone mixed dosing → electrochemical removal oil → inclined tube removal oil → walnut shell filter as shown in Figure 1. The treatment plant works independently with the wastewater containing polymer introduced from a sewage separator outlet pipe confluence of produced liquid process flow during the field test, and it would not affect the existing wastewater treatment process.

The field test applies continuous operation process, and the flow sets 450 L/h. The oil content of wastewater containing polymer with temperature of 60˚C is about 4000 - 5000 mg/L. The added water clarifier is BHQ-04 been using in the platform. And the residence time of wastewater in electrochemical oil-remover and inclined tube separator is both 20 min. Specific process scheme is as shown Table 1.

The water samples collected from different sample connections including imported water sample, electrochemical oil-remover output, inclined tube separator output and walnut shell filter output are used to determine the oil content when the process is stable for 30 min and 60 min respectively. And the filtered water samples are for the index analysis of particle diameter, suspended solid content and turbidity. The analysis methods are based on this standard of 5329 - 2012 SY/T “Water quality standard and practice for analysis of oilfield injecting waters in clastic reservoirs”.

Process One: The oil contents of different sample connections with four different dosages of water clarifier

when the process is stable for 30 min and 60 min respectively are shown in Table 2.

Table 2 shows the remarkable oil removal efficiency when the process with electrochemical technology is stable and the oil contents of the filtered water samples which are clear and transparent are all under 20 mg/L. The slight decrease of the oil content with the increase of electrolysis time reveals the treatment efficiency is jarless when the process is stable. Without the water clarifier BHQ-04, the oil contents of the output of electrochemical oil-remover and inclined tube separator are 73 - 85 mg/L and 64 - 70 mg/L respectively, which are significantly higher than those with water clarifier. Thus it will aggravate the pollution of filter media and operating load. The oil contents of different sample connections are stable along with the increased dosages of BHQ- 04 indicates the good synergistic effect between electrochemical technology and water clarifier, and the oil removal efficiency of electrochemical technology with moderate BHQ-04 (50 - 100 mg/L) more higher.

Process Two: The test results of the process without electrochemical technology are shown in Table 3.

Table 3 shows the oil content and turbidity of water samples of inclined tube separator output present downtrend with the water clarifier BHQ-04 of 150 - 200 mg/L during the process operating without electrochemical technology. The increased dosages of BHQ-04 can probably improve the water quality; however the reagent cost is bound to increase and the rising amount of viscoelastic sludge because of the adding reagent will cause blockage to the following processes.

The comparison of two treatment processes: in process one, the oil content of water samples of inclined tube separator output is 39 - 56 mg/L with the water clarifier BHQ-04 of 50 - 100 mg/L, which is far more less than that of 113 - 210 mg/L in process two. And the filtered water samples with the oil content of 100 mg/L can not reach the injecting water quality standard. The comparative analysis reveals the process with electrochemical technology shows up a better oil removal efficiency. The specific mechanism of electrochemical removal oil in the period of the process operating is free radical (・OH, HO₂・, ・O₂) and strong oxidizing substances (O₃, H₂O₂, ClO⁻) produced on anode electrode can decompose polymer molecules into small molecules; meanwhile the micro bubbles produced on anode and cathode electrode will coalesce the emulsified oil into large oil droplets, at the same time the bubbles also bring the suspended solids and small molecules to the water surface to be removed. The oil removal rate of the complete treatment process forming by electrochemical technology and water clarifier is over 98%, what is more important is the decreased dosages of BHQ-04 reduces the amount of viscoelastic sludge, providing a guarantee for the regular operation of the following process.

The filtered water samples of six different dosages of water clarifier when the two processes are stable for 60min are used for the test of the particle size and suspended solids content to compare the water quality of the two processes.

Results of the particle diameter and median particle diameter of filtered water samples of six different dosages are presented graphically on Figure 2 and Figure 3.

Figure 2 and Figure 3 show 95% particle diameter and median particle diameter of filtered water samples is less than 7.89 μm and equal 2.26 μm respectively with the water clarifier BHQ-04, which are far more less than those of the samples without BHQ-04 in process one; while 95% particle diameter and median particle diameter of filtered water samples increase to 121.5 μm and 87.1 μm in process two. It presents the combination use of electrochemical technology and water clarifier can effectively remove the particulate material in wastewater.

Results of the suspended solid content of filtered water samples of six different dosages are presented graphically on Table 4.

Table 4 shows the suspended solid contents of filtered water samples vary from 13.79 mg/L to 15.52 mg/L with the water clarifier BHQ-04 of 150 - 200 mg/L in process two. And the treatment efficiency of the suspended solid content of 1.39 - 2.04 mg/L with the water clarifier BHQ-04 of 50 - 150 mg/L compared with that of 4.84 mg/L without the water clarifier BHQ-04 in process one indicates that electrochemical technology plays a key role in the treatment of wastewater containing polymer and the aggregation effect with water clarifier for fine particle is very obvious.

The dosage of water clarifier BHQ-04 of 380 mg/L is used for the treatment of wastewater containing polymer in an offshore platform, however the filtered water samples can not reach the injecting water quality standard

and the viscoelastic sludge forming by the adding reagent will cause blockage to the following processes after operating for a period of time. The process of electrochemical technology used in conjunction with BHQ-04 of 50 mg/L can effectually improve the treatment effect of wastewater to meet water quality standards with the power consumption of only 2.0 kw∙h per cubic meter wastewater. The oil content, median particle diameter and suspended solid content of filtered water samples are less than 11 mg/L, 2.26 μm, 2.04 mg/L respectively. The test results show the process has a good economic benefit with 86.8% decrease of agent cost in the case of water quality standards.