Method for treating wastewater or produced water

A method of removing hardness, silica, suspended solids and dissolved solids from a feedwater stream by pretreating the feedwater stream and thereafter directing the feedwater stream through at least one reverse osmosis unit, the method comprising:

• a. removing a majority of the hardness and reducing the concentration of total hardness in the feedwater stream to less than 55 mg/L as CaCO3 during a chemical softening process and also removing silica from the feedwater stream through the chemical softening process by:
• i. directing the feedwater stream into one or more reactors;
• ii. removing a majority of the hardness from the feedwater stream by mixing a first alkaline reagent with the feedwater stream in the one or more reactors wherein mixing the first alkaline reagent causes hardness compounds to be precipitated from the feedwater stream;
• iii. after mixing the first alkaline reagent with the feedwater stream, directing the feedwater stream to a downstream mixing reactor;
• iv. in the downstream mixing reactor directing the feedwater stream into a vertical tube mixer and mixing a flocculant with the feedwater stream and continuously circulating the feedwater stream vertically through the vertical tube mixer and forming floc made up of precipitated solids including precipitated hardness compounds and suspended solid which gives rise to crystallized solids;
• v. downstream of the mixing reactor, separating the feedwater stream into two streams, a concentrated feedwater stream having the precipitated solids including hardness compounds and suspended solids, and a clarified feedwater stream having suspended solids, dissolved solids, and residual hardness;
• vi. recycling at least a portion of the concentrated feedwater stream back to at least one of the upstream reactors and mixing the concentrated feedwater stream with the feedwater stream which gives rise to the further formation of floc and crystallized solid in the mixing reactor;
• vii. wasting at least a portion of the concentrated feedwater stream;
• b. prior to separating the feedwater stream into the concentrated feedwater stream and the clarified feedwater stream, mixing a second alkaline reagent with the feedwater stream and raising the pH of the feedwater stream to above 10.5;
• c. directing the clarified feedwater stream to a filtering unit and further removing suspended solids from the feedwater stream;
• d. directing the clarified feedwater stream into an ion exchange unit and in the ion exchange unit reducing the concentration of total hardness in the feedwater stream to less than 0.5 mg/L CaCO3;
• e. after subjecting the clarified feedwater stream to treatment in the filtering unit and ion exchange unit, directing the clarified feedwater stream to a reverse osmosis unit and removing dissolved solids from the feed stream and producing a permeate stream and a reject stream where the reject stream includes dissolved solids;
• and
• f. generally maintaining the pH at above 10.5 such that when the feedwater stream reaches the reverse osmosis unit the pH of the feedwater stream is above 10.5 which reduces the tendency of one or more membranes forming a part of the reverse osmosis unit to scale.