INTRODUCTION

Wastewater is a broad term that includes both human-generated wastewater (e.g., sewage) and industrial waste streams of water combined with various contaminants from the industrial process. Wastewater typically must be treated to remove at least some of the contaminants before it can be discharged into the environment or reused.

One common technology for the treatment of wastewater is the membrane bioreactor (MBR). A MBR is a technology that combines a membrane filtration process, like microfiltration or ultrafiltration, with a biological wastewater treatment process, such as the activated sludge process. Two MBR configurations exist: internal/submerged, where the membranes are immersed in and integral to the biological reactor and external/sidestream, where membranes are a separate unit process requiring an intermediate pumping step.

MBRs have been shown to be very effective in the treatment of wastewater. In addition, MBRs wastewater treatment plants are simple and inexpensive to operate when compared to other wastewater treatment technologies. MBRs also are known to have a small footprint. MBRs are particularly effective at removing particulate from wastewater.

Membrane materials of membranes used in MBRs may include the following: polyvinylidene fluoride (PVDF); polyether sulfone (PES); polysulfone (PS); polyacrylonitrile (PAN); polyvinylchloride (PVC); polypropylene; chlorinated polyethylene; polyimide; polyamide; cellulose acetate; silicone-coated polyimide; polyether ether ketone (PEEK), alumina, silica, or ceramic, among others. Membranes are typically classified by nominal pore size in microns, with a pore size of 0.0001 micron being typically used in reverse osmosis application up to 0.1 micron pore size, which is commonly used in MBRs and other low pressure filtration applications. The membranes also may have different configurations such as flat sheet, hollow fiber, tubular, and spiral wound to name but a few.

One weakness of the MBR technology is the technology's effectiveness in phosphorus (P) removal. While an essential element for plant life, when there is too much phosphorus in water it can speed up eutrophication (a reduction in dissolved oxygen in water bodies caused by an increase of mineral and organic nutrients) of rivers and lakes which can be extremely harmful to aquatic animal life. For this reason, phosphorus discharge levels are a closely regulated performance criterion for any wastewater treatment plant.

Phosphorus is most commonly found in wastewater in the form of the phosphate ion (PO43−). The phosphate ion PO43−is relatively small and many common phosphate salts (such as salts of sodium, potassium and ammonium) found in wastewater are very water soluble and are difficult to cost-effectively remove using a MBR. This is because the phosphate salts are neither cost-effectively removed by the biological process nor the membrane filtration.

Attempts have been made to improve the phosphorus removal performance of MBRs. Clarifying agents, such as ferric, have been used to precipitate soluble phosphorus and remove it with the solids. However, the current clarifying agents and other techniques that improved the removal of phosphorus by the biological process result in excessive levels of fouling or damage to the membranes in the filtration process. When the membranes become fouled, the MBR operation needs to be halted and the membranes are cleaned or replaced. Using more restrictive membranes that could remove phosphate ions increases the necessary pressure differential across the membrane so much that it makes the pumping costs too excessive to be reasonable. MBRs could be used more commonly and effectively in wastewater treatment if efficient and effective phosphorus removal could be combined with the MBR.

Thus, effective methods of treating wastewater with MBR while removing phosphorus waste are needed.

SUMMARY

As disclosed herein, the present methods are methods of treating wastewater using a membrane bioreactor. These methods include the steps of dosing a wastewater stream, as part of the membrane bioreactor system, containing a first concentration of phosphorus with a dose amount of a rare earth clarifying agent to obtain a dosed wastewater stream, the first concentration of phosphorus being greater than a target concentration of phosphorus, and passing the dosed wastewater stream through the membrane to obtain a membrane permeate stream, the membrane permeate stream having a permeate concentration of phosphorus less than the first concentration phosphorus. In the methods as disclosed herein, the rare earth clarifying agent can be chloride salts of one or more rare earth elements. In certain embodiments, the rare earth clarifying agent can be CeCl3and LaCl3.

In these methods, the rare earth clarifying agent can be used such that the membrane permeate stream has a permeate concentration of phosphorus equal to or less than the target concentration of phosphorus. Additionally, in these methods, the dose amount of the clarifying agent can be set and/or changed based on the permeate concentration of phosphorus, to achieve a permeate concentration of phosphorus equal to or less than the target concentration of phosphorus.

In particular embodiments, the methods as disclosed herein are methods for treating wastewater using a membrane bioreactor including the steps of dosing a wastewater stream, as part of the membrane bioreactor system, containing a first concentration of phosphorus with a dose amount of a rare earth clarifying agent comprising CeCl3and LaCl3to obtain a dosed wastewater stream, the first concentration of phosphorus being greater than a target concentration of phosphorus; removing at least some of the phosphorus as precipitated sludge from the wastewater stream; and passing the dosed wastewater stream through the membrane to obtain a membrane permeate stream, the membrane permeate stream having a permeate concentration of phosphorus less than the first concentration phosphorus.

