PHOTO 1
State-of-the-art odor control facilities will ensure that there are no off-site objectionable odors.

PHOTO 2
Rendering of the upgraded and expanded plant site layout showing plan to preserve forested areas and avoid impacts to sensitive Chesapeake Bay resource protection areas.

PHOTO 3
Aerial photo of York River Treatment Plant site shows forested buffer that was important to maintain during plant nutrient upgrade and capacity expansion.

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Introduction

In 2006 the Hampton Roads Sanitation District (HRSD) faced an aggressive regulatory schedule and complex technical challenge - first, to upgrade the York River Treatment Plant by 2011 to meet "limit of technology" nutrient reduction goals established for tributaries of the Chesapeake Bay by the Virginia Department of Environmental Quality, and second, to double the capacity of the York River Plant from 15 to 30 million gallons per day by 2016 to meet the wastewater treatment needs on the Lower Virginia Peninsula over the next 20 years.

In addition, HRSD needed to provide on-site biosolids disposal facilities in accordance with their District-wide biosolids management plan. Options considered for the York River Treatment Plant included building either an incinerator, dryer-pelletizing facility, or composting facility.

The HRSD-Malcolm Pirnie solution was a step-feed biological nutrient removal (BNR) process for nitrification and partial denitrification, chemical addition for phosphorus removal, denitrification filters to provide "limit-of-technology" nutrient removal (3 mg/l TN and 0.35 mg/l TP), and a biosolids recycle facility.

Integrated Approach

The project addressed the protection of the water, soil and air environments. Treating the wastewater liquid stream to the "limit-of-technology" for nutrient removal will protect water quality of the York River and Chesapeake Bay for future generations. Biosolids will be processed into a product that has been successfully marketed by HRSD as a beneficial soil conditioner and plant food supplement under the Nutri-Green® label for many years.

The project also addressed air impacts by treating odorous air emissions from the preliminary treatment facility building and primary clarifiers. And, to address both indoor air quality and outdoor air emissions associated with the biosolids facility, all air from the enclosed building and air used to aerate the biosolids piles, will be discharged to a biofilter for treatment.

Quality

Based on project results, in December 2007 the York County, VA Board of Supervisors unanimously approved HRSD's plan for upgrade and expansion of the York River Plant. HRSD's General Manager offered this acknowledgment:

"My thanks to you and the MP team for the excellent work in getting us through the permitting process in York County. Have a great holiday! This early gift is making the season bright for me and the future bright for HRSD!"

Providing further savings, a value engineering study resulted in over $3 million in total savings on a life cycle basis. Conducted after the draft preliminary engineering report was completed, the VE study also identified additional VE ideas that will be considered during detailed design that could result in additional savings of $8 million to $10 million.

Originality and Innovation

The selection of deep-bed denitrification filter technology was an innovative approach that drove the entire project. The need to meet the nutrient waste load allocation starting in 2011 created a significant challenge. The York River Plant had to be able to meet an annual total nitrogen (TN) waste load allocation of 274,100 lbs per year at the current plant design capacity of 15 MGD. The plant upgrade had to be able to treat the initial 15 MGD secondary effluent flow, with TN concentrations ranging from 17 to 26 mg/L, to an average annual effluent concentration of 6 mg/L. The selection of deep-bed denitrification filter technology provided the process flexibility to initially treat the high nitrate-nitrogen concentrations and meet the annual TN waste load allocation by 2011. This allowed HRSD to defer the construction of additional BNR reactor volume until the plant expansion to 30 MGD was needed in 2016. This will allow construction to be sequenced more efficiently without multiple contractors onsite.

An innovative centrate treatment system was designed to nitrify and equalize centrate flow. To dewater digested biosolids, HRSD uses centrifuges, which are typically operated 10 hours per day, 4 days per week during nighttime periods of off-peak electrical power rates to save energy costs. During these periods, the centrate, which has an ammonia- nitrogen concentration of about 700 mg/L, is recycled to the head of the plant. Due to the low night time flows and higher detention times in the primary clarifiers, the recycled ammonia-nitrogen accumulates in the primary clarifiers. As the morning plant flow increases, the ammonia-nitrogen accumulated in the primary clarifiers is flushed into the aeration tanks, resulting in increased ammonia- nitrogen in the secondary effluent, complicating the plant's ability to meet "limit of technology" nutrient reduction goals. As a solution to this problem, Malcolm Pirnie designed an innovative centrate treatment system to nitrify and equalize centrate flow prior to recycling it to the main plant for denitrification in the filters.

Complexity

The HRSD-Malcolm Pirnie team was challenged to meet "limit-of- technology" nutrient reduction goals at very low wastewater temperatures. Default values in the BioWinTM process model for kinetic and stoichiometric parameters were initially evaluated using full-scale operating data that indicated a nitrification rate significantly below normal. HRSD and Malcolm and also modified the full-scale facility operating strategy to maintain year- round nitrification. The results of this work changed some process model default parameters critical to process design, saving HRSD about $12 million in biological reactor capital costs.

Social/Economic Advancement

The York River Plant expansion will sustain economic development on the Lower Virginia Peninsula over the next 20 years while protecting water quality in the York River and Chesapeake Bay for future generations. HRSD will recycle about 50,000 pounds per day of biosolids in a beneficial, cost effective, environmentally sound and publicly acceptable manner. State-of-the-art odor control facilities will result in no off-site objectionable odors. Construction of the $400 million project is being phased over an 8-year period to reduce the financial impact on the rate payers by spreading the project cost over several years.