PROCESS FOR CLARIFYING ALGAE-LADEN WASTE WATER STREAM
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for clarifying algae-laden waste water stream to form a waste sludge and an effluent water stream having a lower amount of total suspended solids.
2. Description of the Prior Art
Algae-laden waste water streams are generally very expensive and difficult to clarity to low suspended solids levels. Normal sedimentation techniques (e.g., gravity-settling clarifiers) by themselves will not work with many algae species. The algae exhibits a specific gravity nearly equal to that of water and many species of algae live in the water in a highly dispersed manner. Besides these dispersed species not settling easily, they also may interfere with the settling of other solids,that may be dispersed in water. In the past, it was found that if a coagulation aid was added to an untreated algaeladen waste water stream, coagulation would occur but settling would not rapidly follow.
Rather than directly trying to coagulate suspended solids in algae-laden waste water streams, persons skilled in the treatment of industrial waste water streams first attempted to control algal growth by adding copper sulfate or like chemicals to treatment ponds where algae was produced. See Kirk-Othmer, Encyclopedia of Chemical Technology, Second Ed., Vol. 22, pages 65-82 (1970). However, such chemical control methods have disadvantages. First, the use of chemicals like copper sulfate on a large scale can be expensive. Moreover, the addition of such chemicals to waste water streams introduces still other contaminants to such streams which may have to be removed later on.
Accordingly, there is a need in the art to improve conventional gravity-settling methods for clarifying algae-laden waste water streams without adding any further chemicals to these streams. The present invention is a solution to this need.
BRIEF SUMMARY OF THE INVENTION
The present invention, therefore, is a process for clarifying algae-laden waste water streams, wherein these streams have suspended solids which contain algae. This process comprises the steps of:
(a) deaerating, the waste water stream containing algae to remove at least a portion of the gaseous oxygen attached to the algae in the waste water stream;
(b) passing the waste water stream through a substantially light-free environment for sufficient time to cause at least a major portion of the algae to revert to its nocturnal phase;
(c) then adding a sufficient amount 6f at least one coagulation aid to the waste water stream containing algae in its nocturnal phase to cause coagulation of at least a portion of the suspended solids while in a substantially light-free environment; and
(d) then settling the coagulated solids in the waste water stream in a substantially light-free environment in order to form a waste sludge of coagulated solids and an effluent water stream having a markedly lower amount of total suspended solids.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow chart of one preferred embodiment of the present invention.
FIG. 2 is a detailed view of a reactor clarifier which is a preferred apparatus for clarifying algae-laden waste water streams according to the present invention.
Waste water streams treated by the present invention may be any algae-laden waste water stream from an industrial or municipal operation, or the like. The waste water stream may include either rainstorm runoff, waste water from industrial process streams, sewage and the like, or combinations thereof, which contain an undesirable amount of solids and algae. In the last few years, stricter governmental regulations are requiring the level of Total Suspended Solids (TSS) in the waste water streams which enter public waterways to be lower than what was accepted in the past. Specifically, in at least one case, the TSS of the effluent water stream was limited by governmental regulation to no more than 60 mg. of TSS per liter of water.
For the present invention, waste water streams having more than about 100 mg. of TSS per liter of water of which algae constitutes at least 10% by weight of TSS may be treated. Preferably, it would be acceptable to treat any waste water stream which contained from about 200 mg. to 2,000 mg. of TSS per liter of water. Also, in some instances, it is preferable to have the waste water stream pass through a floating oil boom and a trash screen to remove any appreciable amounts of oil and trash from the stream before this treatment process.
In the first step of the preferred embodiment shown in FIG. 1, the algae-laden waste water stream is cascaded over a waterfall and briefly held in a covered raw water sump (i.e., concrete tank). The waterfall has the effect of deaerating the waste water stream and tends to loosen oxygen bubbles attached to the algae. The waterfall should be of sufficient height to cause a desired amount of deaeration. It has been found that a waterfall of even 2 feet in height is sufficient to cause some deaeration.
After the waterfall, the water stream is held in the sump briefly to allow any entrapped air in the water to be removed. Preferably, the sump is covered to initiate the algae into reverting to their nocturnal phase. However, it should be noted that other means for reducing the free oxygen in the waste water stream besides a waterfall and sump may be herein employed. Centrifuges and the like are some examples of such equivalent means.
After the waste water stream has been held in the sump for the desired time, it is passed through a substantially light-free environment for sufficient time to cause at least a major (i.e., about 50% by weight) portion of the algae contained in the water to revert to their nocturnal phase. The preferred light-free environment is an elongated enclosed zone such as a pipe, although other equivalent means such as tanks may be utilized. In the preferred process, the waste water stream from the sump is pumped through the pipe to a two-section pretreatment tank. A suitable detention time in the pipe may be as little as 15 minutes although it is more preferred to utilize from 20-60 minutes detention time.
The present invention is based on the photosynthesis principle that green plants, like algae, generate oxygen