With the development of offshore oil fields, the treatment of domestic sewage on offshore platforms has become increasingly prominent. In order to realize the “zero discharge” of domestic sewage from the platform, a recycling process for domestic sewage from the offshore platform was proposed. First, the black water of the platform was broken by pipes, pretreatment by anaerobic, then entered the biofilm reactor, and finally reached the index of reclaimed water through micro-electrolysis treatment, and then reused for flushing after passing the treatment. This process can reduce the use of fresh water or sea water on the platform, and avoid the discharge of domestic sewage. At present, it has been applied in an oilfield, and the operation effect is good, and the application prospect is broad.
Offshore platform domestic sewage can be divided into “gray water” and “black water” two types, in which daily bathing sewage, ground washing water and kitchen and laundry room drainage are gray water; Black water from toilet urinal, urinal and infirmary. At present, the main treatment methods of domestic sewage include physicochemical method, biochemical method, electrolysis method and biofilm method. Among them, the dose of disinfectant required by physical and chemical method is large and the size of treatment equipment is large, so it is gradually replaced by biochemical method and electrolysis method. The initial investment of biochemical method is low, the operation cost is low, but the daily operation and maintenance of the treatment device is more complicated. The electrolysis treatment device is small in size, the ions generated by electrolysis can play the role of disinfection and sterilization, and there is no need to add disinfection agents, and there are many kinds of domestic sewage that can be treated, and the treatment device can be started and stopped at any time, and the operation is simple, and the application of offshore platforms is more and more extensive. Biofilm method is a kind of treatment technology combining biochemical method and membrane separation method, which has been applied in onshore oilfield and offshore platform, and has achieved good results. At present, the common treatment process of domestic sewage from offshore platforms is that gray water is simply filtered and enters the domestic sewage treatment device together with black water, and is discharged after the treatment reaches the discharge standard of the relevant sea area. Due to the large proportion of gray water in domestic sewage, in order to reduce the discharge of domestic sewage, some oil fields with water injection demand will treat the qualified gray water back into the formation. However, the COD(Chemical Oxygen Demand) and solid suspended matter content of black water are high, and conventional treatment is difficult to meet the water quality requirements of injection. Therefore, most offshore platforms still discharge black water after treatment.
Design of recycling process of Marine platform domestic sewage
Process flow
In order to solve the problem of black water discharge, this paper proposes to adopt the concept of recycling of domestic sewage when designing the domestic sewage treatment process of offshore platforms. First, the black water is treated to reach the water quality index of the middle water, and then reused for flushing, so as to achieve recycling (FIG. 1).
First of all, the black water enters the anaerobic reactor through the pipeline crusher for pretreatment. The upper and lower vertical baffle flow mode is adopted inside the reactor, which improves the contact area between the black water and anaerobic bacteria, and can acidify and hydrolyze macromolecules and refractory organic matter. Then, the black water enters the biofilm reactor, the reactor is equipped with ultrafiltration membrane group and filler and is divided into several areas, the bottom is equipped with aeration device, the sewage is driven by the air flow in the circulation between different areas, to achieve synchronous decarbonization, nitrogen and phosphorus removal, organic matter is decomposed into CO, HO, sludge after ultrafiltration membrane filtration and sewage separation. The separated water enters the micro-electrolysis device for further processing, and the activated carbon sulfonated coal filler with catalytic activity is introduced into the micro-electrolytic cell (the filler can be used as both an adsorbent and a three-dimensional compound electrode), thus forming countless micro-electrolytic cells, which shorten the swimming distance of the electrolytes, reduce the electrolytic voltage, and greatly reduce the energy consumption and residence time. Micro-electrolysis can effectively remove pollutants such as organic matter, nitrogen and phosphorus in water, and the strong oxidant produced by electrolysis can also play a role in sterilization and disinfection. After the “biofilm + micro-electrolysis” treatment, the black water reaches the reclaimed water reuse standard, enters the reclaimed water pressure tank for storage and reuse, and the sludge separated from the black water is deposited at the bottom of the equipment, which can be regularly cleaned and transported back to the land for treatment.
Non-putty water (washing water, etc.) and putty water (kitchen water, etc.) are treated and filtered respectively into the drainage system of the platform, and then into the production water treatment system, which is used for the injection development of the oilfield after passing the treatment. When the liquid level in the middle water pressure water tank drops or the middle water needs to be replaced, the gray water also enters the domestic sewage treatment device and is used as the middle water after being treated with black water, minimizing the amount of fresh water or seawater.
Main technical features:
In this paper, the recycling process of Marine platform domestic sewage is combined with biofilm and micro-electrolysis technology, which realizes the improvement of traditional domestic sewage treatment process. Compared with the traditional process flow, the Marine platform domestic sewage recycling process proposed in this paper has the following advantages:
1) The reuse of domestic sewage can fundamentally reduce the discharge of domestic sewage from offshore platforms and protect the Marine environment;
2) Using reclaimed water for flushing can reduce the amount of seawater used for flushing in living buildings and reduce the load of sea water pumps;
3) Using reclaimed water to replace seawater for flushing can reduce pipeline corrosion; 4) The remaining sludge after domestic sewage treatment is less, which is easy to handle and transport.
Application in a specific oil field
The recycling process of domestic sewage from offshore platform proposed in this paper has been applied to two platforms in an oil field, and the designed treatment scale is 60 people. From the field application effect of platform A, the process and equipment run stably, and the treated domestic sewage can meet the water quality requirements of reclaimed water (Table 1). Moreover, after the black water treatment of the platform is qualified, most of it is recycled as reclaimed water, but there is still an outlet port in the process, and “zero discharge” is not fully realized. Platform B is a newly built central treatment platform in the oilfield, which is located in the sensitive area of ecological environment. According to the requirements of environmental protection, the domestic sewage produced by the platform is not allowed to be discharged, so the domestic sewage treatment process of the platform has been improved. In the improved domestic sewage treatment process, there is no longer an outlet port, but a sewage recovery tank. When the water needs to be replaced or emergency conditions occur, the reclaimed water can be discharged into the production sewage treatment and reinjection system, or the reclaimed water or sewage can be discharged into the sewage recovery tank and regularly pulled back to the land for treatment.
conclusion
It can be seen that the recycling process of Marine platform domestic sewage proposed in this paper enables the Marine platform domestic sewage system to achieve true “zero discharge”, which can make full use of platform water resources, reduce domestic sewage discharge, protect the environment, and have broad application prospects.
1) In order to solve the problem of domestic sewage treatment and utilization on offshore platforms, a recycling process for domestic sewage on offshore platforms is proposed. On the basis of the traditional treatment process, combined with the “biofilm + micro-electrolysis” treatment technology, the black water treatment reaches the water index, and is reused for flushing, effectively reducing the use of fresh water or seawater on the platform, and avoiding the discharge of domestic sewage.
2) At present, the process has been applied in 2 platforms of an oilfield, the on-site process and equipment run stably, and the water quality after treatment can meet the standard requirements. The improved process adopts waste water recovery tank instead of traditional sea port, which can realize “zero discharge” of domestic sewage.
The article is reproduced from China Offshore Oil and Gas, Volume 31, Issue 4, August 2019