Sunday, 5 July 2015

Sewage Treatment Package Plant Problems

I have written before about the many examples of failure of packaged sewage treatment plants I have been called on to troubleshoot. I have also had students carry out investigations based in scientific literature. The outline of one such investigation follows:

Failures in Packaged Plants result in homeowners or developers spending considerable sums for their repair, upgrade or change; these failures mostly result in an effluent that does not conform to the Environment Agency’s consents, though there can also be odour problems. I have investigated the causes of these failures and the practices, methods and policies which can be adopted to reduce or eradicate these problems.

A list of Package Plants have been approved by the Environment Agency which indicates they should achieve the consent levels to attain an effluent quality fit for disposal in the environment. Unfortunately, a significant minority of these plants fail to meet the Environment Agency’s consents. In some instances minor repairs are required, but in other cases the plant installed is later found to be unsuitable and a replacement is required.

Homeowners are most often the ones who incur additional unnecessary expense for a plant that should be functioning and in some instances they may have the additional expense of paying fines. Are the owners at fault for poor maintenance or is their limited or lack of knowledge about Package Plants the root cause to these problems? Is the retailer’s team poorly skilled regarding design or installation? Can the Environmental Agency set more stringent rules to reduce package plant problems? Are there any other causes and what can be done?

In the United Kingdom centralized large-scale sewage treatment works service urban areas where the community is connected via mains. Outside these densely populated areas, decentralized non-mains treatment methods are used. These include cesspools, septic tanks, constructed wetlands, and package treatment plants.

Cesspools are watertight enclosures that store, but do no treat, domestic wastewater. They do not have an effluent discharge into the environment but require frequent emptying by approved companies using suction tankers.

Septic tanks, unlike cesspools, are primary treatment tanks that offer some degree of wastewater treatment with anaerobic bacterial action in a closed sedimentation tank. The treatment however is limited and as such the effluent discharged requires further treatment in a drainage field. Even with the addition of the drainage field, the effluent treatment is not satisfactory if a watercourse is in the vicinity. Other limitations include the availability of enough land to accommodate a drainage field, and additionally the characteristics of the soil should be sufficiently permeable to allow proper filtration and prevent clogging, while the water table should be sufficiently deep to prevent its contamination.

Constructed wetlands are a natural means of treating settled wastewater or septic tank effluent which is introduced to reed beds either via vertical or horizontal flow. It is an alternative to conventional secondary treatment. Such systems are area-extensive, needing sufficient space to support reed beds that will adequately treat the wastewater.

A Packaged Plant is an improved option for wastewater management in rural areas compared to cesspools, septic tanks, and constructed wetlands as it offers a high effluent quality with use of limited land area although most packaged plants require electricity to function. The various types of wastewater treatment packages vary in their capabilities to discharge directly into a watercourse, whether it is necessary to obtain consent for effluent disposal into controlled waters from the regulatory agency, the frequency of desludging, proximity to dwelling and wells or boreholes, and the need for a power supply.

Package plants are prefabricated units, which may in some cases be installed and ready for commissioning within a day. It is the preferred option by the regulating agencies in the UK, especially where conditions are unfavourable, requiring high effluent quality, and in some cases where restrictions are applicable it is the only option. Package treatment plants can produce effluent qualities of either secondary or tertiary quality depending on the type of plant, which may use mechanical/biological processes as well as chemical precipitation.

The types of Package Plants include rotating biological contactors (RBCs), sequencing batch reactors (SBRs), biological aerated filters (BAFs), submerged aerated filters (SAFs) and membrane bioreactors (MBR’s).

MBR’s produce very high removal efficiencies of tertiary quality. However, the elevated cost of the system as well as high maintenance costs limits its use since it is mostly uneconomical . For these reasons it is not one of the package plants recommended by the Environmental Agency and is not part of this study.

RBC package plants use aerobic action to treat wastewater. They consist of rotating plastic medium, which has about 40% of its depth submerged in wastewater and the other 60% exposed to the air. As the medium rotates it alternatively makes contact with the wastewater and the air, which supply organic pollutants and nutrients from the former, and oxygen from the latter, both of which are needed for the growth of the biomass on the medium. The wastewater becomes treated through the processes of biological oxidation and biological flocculation.

The BAF or SAF package plants provide secondary wastewater treatment using a full-scale treatment process contained in one unit. Wastewater directly enters the system from the dwelling into a primary settlement tank. The settled wastewater progresses to the submerged plastic medium aeration basin used for biomass growth. Air is supplied at the bottom of the basin to supply oxygen to the biomass. The secondary treated effluent from the aeration basin continues to a secondary settlement chamber to allow sludge to settle and the final effluent to be discharged.

In the UK, British Water is the trade organization which represents the stakeholders of the water industry, including government, regulating bodies, institutions, consumers and the media. One of this organization’s responsibilities is to disseminate the codes of practise to be adopted for small wastewater treatment plants including package plants.

These guidelines, if followed should avoid failures in package plants. The codes of practise therefore play a key role in defining, identifying, and solving failures of package treatment plants.

The codes of practise include:

“Code of Practice - Flows & Loads 4 - Sizing Criteria, Treatment Capacity for Small Wastewater Treatment Systems” 

This guideline focuses on the design and sizing of a package plant. It provides the sewerage loadings and daily flow rates, the nature and source of the sewerage to be treated for various types of dwellings. This information allows the contractor to select the correct sizing of the plant (Water, 2009)

“Code of Practice - Guide to the Installation of Small Wastewater Treatment Systems” 

This code of practise, useful to both the contractor and the owner, serves as a checklist to outline the installation requirements and the stages necessary to install package plants.

“Code of Practice – Guide to the Desludging of Small Wastewater Treatment Systems” 

These guidelines enable the owner to understand how package plants work, why sludge is formed and the importance of sludge removal.

“Code of Practice – A Guide for Users of Small Wastewater Treatment Systems” 

This code of practise will educate owners and users about how package plants operate and the conditions the plants must meet to satisfy regulators consents (Water).

“Code of Practice – Maintenance and servicing of Small Wastewater Treatment Systems (Package Plants) up to 50 Population Equivalents (PE) and Larger Systems up to 1000” 

In these guidelines the practices to be adopted for maintaining and servicing the Package Plants are listed. As previously seen, British Water provides guidelines throughout the lifecycle of the package plants, from their design to their operation and maintenance.

Dee and Sivil identified potential problems in package plants which include the following:
  • poor design due to insufficient plant capacity to support organic and nitrogen loads or the wastewater flow. 
  • Fluctuations of wastewater resulting in hydraulic shock loads 
  • A wide and sudden variance in wastewater temperature affects the oxidation of ammonia, particularly with drops in temperature. 
  • Build-up of grease and fat in the plant 
  • Build up of solids in pump wells
  • A very high volume of commercial wastewater which may be highly toxic and produce a final effluent which does not meet the Environmental Agency’s consents.

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