Technologies Used for Wastewater Treatment and Reuse

In this article we will discuss about the appropriate technologies used for wastewater treatment and reuse.

G. Tchobanoglous

University of California Davis CA, USA

Abstract:

During the past 15 years, a wide variety of technologies have been developed for the treatment of wastewater for reuse applications To assess the need for alternative technologies, potential reuse applications are reviewed. Changes in wastewater management, including the growing importance of decentralized wastewater management, are examined.

The role of decentralized wastewater management is considered with respect to wastewater treatment, reclamation, and reuse. Technologies are discussed for a wide range of reuse applications, from large centralized systems to individual homes. The performance of representative technologies is assessed. A brief overview of wastewater reuse in the United States (US) is also presented.

Introduction:

The importance of wastewater reclamation in the field of water resources management is now commonly acknowledged. Similarly, its important role in maintaining a sustainable environment is also gaining wider acceptance. Recognizing its many applications and benefits, the purposes of this paper are fourfold.

Firstly, to review potential reuse applica­tions to provide a frame of reference for the effluent quality and the degree of reliability that may be required, and to identify issues and impediments to implementation. Secondly, to review changing concepts in managing wastewater and the impact of these changes on wastewater reclamation. Thirdly, to review appropriate technologies for wastewater treat­ment and reuse. The fourth purpose is to present a brief overview of wastewater reuse in the US.

The material presented here is an introduction to these topics. Specific details will be presented throughout the conference. Further, because so much has been written about large scale reclamation facilities, this paper focuses on decentralized systems.

Reuse Applications:

In planning and implementing wastewater reclamation and reuse, the reuse application (Table 1) will usually govern the wastewater treatment needed, and the degree of reliability required for the treatment processes and operations. Because wastewater reclamation en­tails the provision of a continuous supply of water to consistent quality, the reliability of the treatment processes and operations must be evaluated in the planning stage.

Specific reuse categories and treatment technologies that may be applicable will depend on the location and type of wastewater management employed (e.g., centralized versus decentralized, discussed subsequently.) Worldwide, the most common use of reclaimed wastewater has been for agricultural irrigation. Recently, groundwater recharge and indirect and direct potable reuse have received considerable attention in the US.

The re-purification project in San Diego, CA, in which it is proposed to blend re-purified wastewater with local runoff and imported water in a local water supply storage reservoir, is an example of such a project.

Decentralized Management:

In the US and elsewhere, it has become apparent that it may not be possible, due to both geographic and economic reasons, to provide sewerage facilities for all of the residents either now or in the future. As a result, the focus of wastewater management is changing from constructing and managing regional sewerage systems to constructing and managing decen­tralized wastewater treatment facilities.

Decentralized wastewater management (DWM) may be defined as the collection, treatment, and disposal/reuse of wastewater from individual homes, clusters of homes, or isolated communities, industries, or institutional facilities. Decentralized systems maintain both the solid and liquid fractions of the wastewater near their point of origin, although the liquid portion may be transported to a centralized point for further treatment and reuse.

Given that complete sewerage will not be possible in the foreseeable future, and that increasing demands are being made on freshwater supplies, it is clear that DWM is important in developing long-term strategies to manage our environment. As more emphasis is placed on DWM, the opportunities for localized reclamation/reuse will increase. Thus, in addition to large scale reclamation projects, it will become important to develop strategies and operating agencies for localized reclamation projects.

Appropriate Technologies:

Technologies to manage wastewater will vary depending on whether centralized or decentralized wastewater systems are used. In general, decentralized systems will be used for un-sewered areas, for localized reclamation, and satellite treatment. Treatment technolo­gies for centralized systems are not considered in detail here.

The focus of the following discussion in on the options and technologies for un-sewered areas. The performance of selected wastewater treatment technologies for centralized and decentralized systems is also considered.

Centralized Systems:

For centralized systems, treatment technologies to remove conventional contaminants (e.g., biochemical oxygen demand and suspended solids) are typically based on the use of the suspended growth (e.g., activated sludge) or attached growth (e.g., trickling filter) processes.

Depending on the required effluent quality, effluent filtration has become commonplace, especially where UV disinfection is to be used. Tech­nologies commonly used for centralized systems may be found in standard textbooks. When indirect and direct potable reuse is being considered, some form of advanced treatment will be required. Typical examples of the types of technologies that have been evaluated in several large scale reuse test programs are illustrated in Fig. 1.

Fig. 1: Process flow diagrams for various levels of advanced wastewater treatment.

A new innovative filter involving the use of a synthetic fiber filter medium is now being tested for Title 22 reclamation applications at the University of California at Davis. The filter is unusual in a number of ways: the filtering medium is highly porous, the porosity (void ratio) of the medium can be modified, to backwash the filter, the porosity (size) of the filter bed is increased mechanically, and the filter operates at very high filtration rates [e.g., 400 to as high as 1600 L/m²*min (10 to 40 gal/ft^2*min)].

In addition to the filtration of secondary and advanced treatment effluents, potential uses of this technology include the filtering of primary effluent and storm flows. A treatment process of wide interest, especially in conjunction with the use of medium energy UV disinfection, involves the use of conventional or enhanced primary sedimentation followed by high-rate filtration.

Use of this technology for the filtration of the wastewater from Mexico City, which has received primary treatment, is currently under investigation. The main concern in this investigation is the control of worms.

Un-Sewered Areas:

To protect the environment and to maximize reuse opportunities, discharge requirements for treated wastewater for small dischargers are now the same as those for large dischargers. The challenge is to be able to provide the required level of treatment in decentralized systems, subject to serious economic constraints. Alternative wastewater management technologies for un-sewered areas are reported in Table 2.

Individual Systems:

Wastewater from individual dwellings and community facilities in un-sewered locations is usually managed by onsite treatment and disposal systems. Although a variety of onsite systems have been used, the most common system consists of a septic tank for the partial treatment of the wastewater, and a subsurface disposal field for final treatment and disposal of the septic tank effluent.

Two noteworthy developments in pre-treatment include the development of water-tight structurally-sound septic tanks and the effluent filter vault. Use of the effluent filter vault to elimi­nate the discharge of untreated solids has significantly improved in the quality of the effluent discharged from septic tanks.

The development of the effluent filter vault is also significant because it has made the use of small high-head (e.g., 100 m, 300 ft) multi-stage well pumps feasible, for pressure dosing of soil absorption systems and pressure sewers.

Final treatment and disposal of the effluent from a septic tank or other treatment unit is accomplished, most commonly, by means of subsurface-soil absorption. A soil absorp­tion system, commonly known as a leach-field, consists of a series of trenches [0.9 to 1.5 m (3 to 5 ft)] filled with a porous medium (e.g., gravel).

Effluent from the septic tank is applied to the disposal field by intermittent gravity flow, or by periodic dosing using a pump or a dosing siphon. Unfortunately, conventional trench designs fail to take maximum advantage of the treatment capabilities of the soil because they are typically located below the region of maximum bacterial activity.

New trench designs now use very shallow trenches with no porous medium (Fig. 2). Using shallow trenches enhances the treatment of the effluent with respect to the removal of BOD, SS, phosphorus, and nitrogen. It is interesting to note that using shallow trenches was recommended in a Public Health Bulletin in 1915.

Fig. 2 : Typical shallow unfilled leach-field.

Because conventional disposal fields cannot be used in some locations, many alterna­tive systems have been developed. The most successful include intermittent and recirculating granular-medium filters. Intermittent sand filters have become quite popular for single family residences, in all parts of the US, because of their excellent performance, reliability, and relatively low cost.

The effluent from an intermittent sand filter is of such high quality it can be used in a variety of reuse applications, including drip irrigation. Recirculating sand and granular-medium filters are used for larger flows. A variety of anaerobic, aerobic, and combined anaerobic/aerobic systems have also been developed. However, the use of aerobic biological treatment systems without the availability of a responsible management agency is not recommended.

For enhanced removal of nitrogen, septic tanks with integral trickling filter or absorbent highly porous plastic elements have been developed. Complete recycle systems have been developed for commercial buildings where the treated effluent is used for toilet and urinal flushing or for landscape irrigation. Where an acceptable onsite disposal system cannot be installed, holding tanks can be employed.

Cluster and Community Systems:

The treatment component of cluster and commu­nity systems will vary with the size of the installation and the final disposition of the effluent. Typically, a large septic tank will be used for a cluster of homes. Imhoff tanks, previously used, are making a comeback in modified forms.

In some communities, septic tanks may be used for the separation of settle able solids and greases and oils. Recirculating granular-medium filters are used in conjunction with septic tanks where a higher level of treatment is required. Pre-engineered and constructed ‘package’ plants, and individually designed plants, are used where the flows are larger.

The methods used for effluent disposal will also vary with the size of the system and the local reuse opportunities. For small installations serving a cluster of homes, effluent disposal is most commonly accomplished using disposal fields.

As the size of the system increases, the methods used for the disposal of effluent are, essentially the same as those used for larger systems (Table 2). Disinfection, where required, is by means of chemical addition (e.g., chlorine and related compounds) or ultraviolet (UV) radiation.

Performance Expectations:

Performance expectations for various technologies are important with reclamation and reuse applications. Treatment levels achievable with vari­ous combinations of unit operations and processes used for wastewater treatment, are reported in Table 3. Treatment technologies for individual and small systems are reported in the last four lines of Table 3.

What is interesting to note, is that many of the treatment systems used for individual homes, cluster systems, and small systems perform at a level equal to or better than comparable large scale centralized systems. Where effluent disinfec­tion is required, the use of chlorine and related compounds and conventional low pressure UV radiation are used. Clearly, technology is now available to produce high quality water from wastewater, regardless of system size.

Reclamation in the US:

The seven principal categories of municipal wastewater reuse in the US, are listed in the Table 1. Agricultural and landscape irrigation are by far the largest uses. Although potable reuse is the least used, indirect and direct potable reuse have recently received the greatest amount of attention in US, especially in water short areas. To assess the available technology, a number of advanced treatment process flow diagrams have been studied (Fig. 1).

In San Diego, CA, the quality of water produced from an advanced water treatment (AWT) process was compared to one of the local water supply sources. It was concluded that “The health risk associated with the use of AWT water as a raw water supply is less than or equal to that of the existing City raw water as represented by the water entering the Miramar water treatment plant”.

Similar findings have been reported in other studies. How to integrate the use of re-purified water in the overall management of water resources is the issue that must be addressed, if the field of wastewater reclamation is to advance.