Municipal wastewater refers to wastewater that is discarded from households. Also referred to as sanitary sewage, such water contains a wide variety of dissolved and suspended impurities. It amounts to a very small fraction of the wastewater by weight. But, it is large by volume and contains impurities such as organic materials and plant nutrients that tend to rot. The main organic materials are food and vegetable waste, human waste, plant nutrient, organic waste from soaps, washing powders, etc.

This section provides details on the latest developments and efforts in the municipal wastewater treatment.

We have discussed the following:

·    Current Municipal Wastewater Treatment Process

·    New Technologies in Municipal Wastewater Treatment

o  Municipal Wastewater Treatment Using the Anaerobic Fluidized Bed Reactor (AFBR).

o  Pilot Plant Study of Bioflocculation in the Dual Trickling Filter/Solids Contact (TFE/SC) Process.

o  AGAR Process

o  Electrochemical Technologies in Wastewater Treatment

o  Solar Photocatalytic Treatment of Synthetic Municipal Wastewater

o  Characterization and Fate of N-Nitrosodimethylamine Precursors in Municipal Wastewater Treatment Plants

o  Impact of Colloidal and Soluble Organic Material on Membrane Performance in Membrane Bioreactors for Municipal Wastewater Treatment

Current Municipal Wastewater Treatment Process

Conventional sewage treatment may involve three stages, called primary, secondary and tertiary treatment.

Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.

Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.

Tertiary treatment is sometimes defined as anything more than primary and secondary treatment. Treated water is sometimes disinfected chemically or physically (for example by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

New Technologies in Municipal Wastewater Treatment

The article below demonstrates a project conducted by the Urban Waste Management and Research Center (UWMRC) which has two components subprojects:

·    Municipal Wastewater Treatment Using the Anaerobic Fluidized Bed Reactor (AFBR).

·    Pilot Plant Study of Bioflocculation in the Dual Trickling Filter/Solids Contact (TFE/SC) Process.

Municipal Wastewater Treatment Using the Anaerobic Fluidized Bed Reactor (AFBR)

The main objective of this project is to demonstrate the technical feasibility of providing secondary municipal wastewater treatment using the following treatment sequence: grit removal, fine screening, AFBR, flocculation, and final clarification, to provide adequate and stable chemical oxygen demand (COD), and suspended solids (SS) removal.¹

Pilot Plant Study of Bioflocculation in the Dual Trickling Filter/Solids Contact (TFE/SC) Process

The main objective of the project is to study the factors affecting the performance of the solids contact chamber (SCC) with regard to both bioflocculation and substrate removal efficiency. The research plan has two phases. The first phase consisted of feeding primary effluent to the TF/SC pilot plant and to observe the effect of operating variables on the plant performance. In the second phase, which is ongoing, the primary effluent was substituted for the effluent of a rotating fine screen. With the use of a 0.020 inch wedge wire rotating screen instead of a primary clarifier, the sewage being fed to the TF/SC system has a lower quality, thus creating higher organic and solids loadings. This effluent was fed directly to the solids contact chamber to test the ability of this unit to handle alone all of the COD contained in this wastewater stream. The general objective of this phase is to establish the ability of the SCC to operate under the higher organic and solids from “poor quality” influent. ²

AGAR Process

The AGAR² process (Attached Growth Airlift Reactor) is a flexible process, which can be applied in a number of main plant configurations according to existing conditions and plant requirements. Once the desired process configuration has been decided upon and the required civil and electromechanical works completed, Aqwise Biomass Carriers are introduced into the tanks in a quantity calculated for the required loads. Additional increases in treatment capacity can be achieved simply and economically simply by adding more plastic carriers to the process. The basic configurations applied in municipal wastewater treatment are:

·    Moving Bed Bio Reactor (MBBR) configuration

·    Fixed Film Activated Sludge Treatment (FFAST) configuration

·    Integrated Fixed-film Activated Sludge (IFAS) configuration

·    The AGAR Roughing Filter for Biological Nutrient Removal (RF-BNR) configuration offers a treatment process based on a bio-film which provides organic carbon removal, followed by nitrification and de-nitrification, without circulation of activated sludge. Denitrification occurs in both pre-denitrification reactor, and in an innovative, patent pending, fixed/moving bed reactor in which intensive endogenous denitrification occurs.

 Benefits

·    Minimal or no additional reactor volume required.

·    Minimal change to plant operations

·    Minimal interference during the upgrade

·    Staged implementation

·    Withstanding of Toxic or Shock loads of industrial wastewater

 Electrochemical Technologies in Wastewater Treatment

EC has been in use for water production or wastewater treatment. It is finding more applications using either aluminum, iron or the hybrid Al/Fe electrodes. The separation of the flocculated sludge from the treated water can be accomplished by using EF. The EF technology is effective in removing colloidal particles, oil & grease, as well as organic pollutants. It is proven to perform better than either dissolved air flotation, sedimentation, impeller flotation (IF). The newly developed stable and active electrodes for oxygen evolution would definitely boost the adoption of this technology. Electrooxidation is finding its application in wastewater treatment in combination with other technologies. It is effective in degrading the refractory pollutants on the surface of a few electrodes. Titanium-based boron-doped diamond film electrodes (Ti/BDD) show high activity and give reasonable stability. Its industrial application calls for the production of Ti/BDD anode in large size at reasonable cost and durability.³

Solar Photocatalytic Treatment of Synthetic Municipal Wastewater

The photocatalytic organic content reduction of a selected synthetic municipal wastewater by the use of heterogeneous and homogeneous photocatalytic methods under solar irradiation has been studied at a pilot-plant scale at the Plataforma Solar de Almeria. In the case of heterogeneous photocatalysis the effect of catalysts and oxidants concentration on the decomposition degree of the wastewater was examined. By an accumulation energy of 50 kJ L⁻¹ the synergetic effect of 0.2 g L⁻¹ TiO₂ P-25 with hydrogen peroxide (H₂O₂) and Na₂S₂O₈ leads to a 55% and 73% reduction of the initial organic carbon content, respectively. The photo-fenton process appears to be more efficient for this type of wastewater in comparison to the TiO₂/oxidant system. An accumulation energy of 20 kJ L⁻¹ leads to 80% reduction of the organic content. The presence of oxalate in the Fe³⁺/H₂O₂ system leads to an additional improvement of the photocatalytic efficiency.⁴

Characterization and Fate of N-Nitrosodimethylamine Precursors in Municipal Wastewater Treatment Plants

The potent carcinogen, N-nitrosodimethylamine (NDMA), is produced during disinfection of municipal wastewater effluent from the reaction of monochloramine and organic nitrogen-containing precursors. To delineate the sources and fate of NDMA precursors during municipal wastewater treatment, NDMA formation was measured after extended chloramination of both model precursors and samples from conventional and advanced wastewater treatment plants. Of the model precursors, only dimethylamine, tertiary amines with dimethylamine functional groups, and dimethylamides formed significant NDMA concentrations upon chloramination. In samples from municipal wastewater treatment plants, dissolved NDMA precursors always were present in primary and secondary effluents. Biological treatment effectively removed the known NDMA precursor dimethylamine, lowering its concentration to levels that could not produce significant quantities of NDMA upon chlorine disinfection. However, biological treatment was less effective at removing other dissolved NDMA precursors, even after extended biological treatment. Significant concentrations of particle-associated NDMA precursors only were detected in secondary effluent at treatment plants that recycled water from sludge thickening operations in which dimethylamine-based synthetic polymers were used. Effective strategies for the prevention of NDMA formation during wastewater chlorination include ammonia removal by nitrification to preclude chloramine formation during chlorine disinfection, elimination of dimethylamine-based polymers, and use of filtration and reverse osmosis to remove particle-associated precursors and dissolved precursors, respectively.⁵

Impact of Colloidal and Soluble Organic Material on Membrane Performance in Membrane Bioreactors for Municipal Wastewater Treatment

Two parallel membrane bioreactors (2 m³ each) were operated over a period of 2 years. Both pilots were optimised for nitrification, denitrification, and enhanced biological phosphorous elimination, treating identical municipal wastewater under comparable operating conditions. The only constructional difference between the pilots was the position of the denitrification zone (pre-denitrification in pilot 1 and post-denitrification in pilot 2). Despite identical modules and conditions, the two MBRs showed different permeabilities and fouling rates. The differences were not related to the denitrification scheme. In order to find an explanation for the different membrane performances, a one-year investigation was initiated and the membrane performance as well as the operating regime and characteristics of the activated sludge were closely studied. MLSS concentrations, solid retention time, loading rates, and filtration flux were found not to be responsible for the different performance of the submerged modules. These parameters were kept identical in the two pilot plants. Instead, the non-settable fraction of the sludges (soluble and colloidal material, i.e. polysaccharides, proteins and organic colloids) was found to impact fouling and to cause the difference in membrane performance between the two MBR. This fraction was analysed by spectrophotometric and size exclusion chromatography (SEC) methods. In a second step, the origin of these substances was investigated. The results point to microbiologically produced substances such as extracellular polymeric substances (EPS) or soluble microbial products (SMP).⁶