SEATTLE1 KING COUNTY STUDY

L 9 COLLECTING i AND

COMPOSTING FOOD

B RESIDUALS

i

OOD residuals were collected from three dozen businesses and com- posted during a study conducted by Seattle and King County, Washing- ton. The collection and composting trials involved the participation of a representative sample of genera-

tors. Materials were picked up by a local hauler and taken to a private site where a pi- lot food residuals composting facility was de- veloped and operated. Afterwards, the fin- ished compost was tested and evaluated.

The $650,000 “Commercial Food Waste Composting Demonstration Project” illus- trates the feasibility of organics diversion and provides guidance for future projects. The following report - which is number two of a three part series on the project - de- scribes the methods used, challenges en- countered and observations made during the operational stage of the study. The first part, “Measuring Generation of Commercial Food Residuals,” appeared in the July, 1995, Bio- Cycle, and focused on a generator study. The series will conclude next month with an eco- nomic analysis of food residuals composting.

COLLECTING MATERIALS Food trimmings were collected from three

categories of generators: wholesale and re- tail food distributors; food service establish- ments and restaurants; and food product processors. The 36 participants were select- ed to provide quantities of food residuals in proportion to the generation rate deter- mined in the generator study. Participants were located, by design, in a variety of geo- graphic areas in King County, ranging from downtown Seattle to primarily residential neighborhoods with a few small businesses.

The materials collected included all major classes of food including vegetables, fruits, meat, fish, dairy and bakery residuals. They included certain organic materials (ice cream

A study focuses on the practical challenges and solutions of managing a commercial organics composting operation.

Part 11

Gerfy Crotelzzr and Rob Steuteville

Grocery store employee places source separated produce scraps into a disposal bin for collection (top). A cart is placed in position for dumping into the collection vehicle (below).

and fish scraps, for example) that posed par- ticular challenges from a collection and com- posting standpoint. However, collection oc- curred in the winter so that potential odor problems in containers and vehicles were minimized. The organic materials contained some inert contaminants such as bottles, cans, plastic film and a few bags of refuse. These could theoretically be kept to a mini- mum through education and enforcement a t the collection point. The delivered food trim- mings were accompanied by a considerable amount of liquid, requiring management in transport and at the receiving location.

The collection occurred over a 10 day pe- riod during January and February, 1993. A total of 34 tons was picked up - 33 per- cent from food service establishments or restaurants (21 participants), 56 percnnr from food wholesalers and retailers i 11 par- ticipants), and 11 percent from food proces- sors (four participants).

The wholesale/retail category, which in- cludes supermarkets and their suppliers. generated the most commercial organics (an average of 1.7 tons/establishment). That group was followed by the processors (near- ly one todestablishment). The restaurant and food service category (0.5 tonshusiness) proved the most difficult to motivate. Orer- all, the restaurants were able to recover from one-third to two-thirds of their estimated weekly generation. The reasons for the low separation rate varied among the restau- rants, but typically collection from certain areas, e.g. near food servers or dishwa-h 3 e n . was difficult to institute for just one week. Recovery of food scraps from kitchen areas was generally more feasible. Among the restaurantdfood service establishments, three out of 21 did not provide residuals a t all during the collection period.

No special containers were provided to col- lect food trimmings inside the businesses. Each provided their own containers for 3 s many collection points as they deemed nee- essary. The participants were encouraged to use existing containers or five gallon buckets to facilitate food separation from as many 10- cations as possible.

Firms were instructed not to line contain- ers with plastic bags as they normally do in

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the interest of keeping feedstock as clean as possible. With only one exception, this instruction was followed. Five busi- nesses received kraft or kraft-cellophane bags as a substitute for plastic. In general, the kraft bags leaked more than the kraft-cello bags. Furthermore, the bags were not the right size for existing indoor collection containers, but they did help to keep the bins cleaner. It was not clear from this brief trial whether bags offer advantages that justify their cost.

All material collected indoors was taken by employees of each business to an outdoor container or containers, provided by the hauler. Dumpsters or a combination of wheeled carts were used for most businesses. The 90 gallon wheeled carts had an unintended benefit - they could be set up inside and then rolled outside for collection. Three manufacturers, Zarn, Roto Injection and Rehrig Pacific, donated carts for use dur- ing the event. One firm, a large produce wholesaler, received a 20 cu.yd. rolloff container for one time pickup of residuals.

In general, collection containers and vehicles used by the hauler, Emerald City Recycling of Seattle, performed well. Some leakage of liquids from the collection containers was ob- served - primarily from the wheeled carts. With one excep- tion, leakage from dumpsters was not observed. Leakage from the collection trucks occurred during the morning of the first day, but stopped after the seal on the back hatch was tightened. No odor problems were noted during the collection event. Most of the businesses received two collections.

SITE AND MATERIAL PREPARATION A pilot scale, temporary composting facility was designed,

permitted, constructed, operated and decommissioned as part of the study. The location of the facility at the Cedar Grove Composting Company site in Maple Valley, eastern King County, was a major factor in success of the project, be- cause it helped to resolve many of the permitting, construc- tion and operating issues. The temporary facility was de- signed to control potential adverse effects such as odor generation, pest attraction and leachate generation.

Two processing technologies were compared: aerated static pile (ASP) and aerated turned windrow (ATW). Three bulking materials were examined: yard trimmings; yard trimmings with mixed paper; and wood waste (equal volumes of chipped pallets and sawdust). The equipment and operators involved in preprocessing, mixing and pile preparation were provided by Cedar Grove. Feedstock storage and pile mixing were con- ducted on an outdoor concrete pad located near the compost- ing buildings. Heavy equipment included a hammermill, front-end loader and Scarab windrow turner.

With the exception of the sawdust component of the wood waste, all of the bulking materials were size reduced with an on-site hammermill. The mixed paper was incorporated into the yard trimmings before grinding to facilitate more ho- mogenous size reduction. The wooden pallets were shredded twice in order to fully break down the larger pieces. The bulk- ing materials were processed several days before the delivery of the food residuals.

The food trimmings were delivered in six loads on two days. Upon delivery, the materials were deposited up gradient of a yard debris swale (a depression engineered to direct runoff), positioned to absorb liquids from the food scraps. The deliv- ery was inspected and large contaminants, such as plastic bags containing refuse, were removed. The pile of organic ma- terials was mixed with a front-end loader to produce a more homogenous feedstock.

On the first delivery day, the residuals were divided into three piles, each combined with a different bulking agent in a 3:l ratio, the smaller portion being food trimmings. The same procedure was followed the second delivery day, and then the piles with similar bulking agents were combined and turned three times.

The three piles were further divided into two equal piles - one for each composting process. The six piles had an initial

78 BIOCYCLE AUGUST 1995

volume of 30 cu.yds. each. They were trans- ported separately into two composting build- ings (one built for each composting process) using a front-end loader. Piles were then re- constructed to completely cover the aeration ditches (which were filled with wood chips). A one foot thick layer of oversize materials screened from yard trimmings compost was used as an insulative cover on the ASP piles, increasing their volumes to approximately 55 cu.yds.

COMPOSTING TRIALS The initial mixtures had near optimal

bulk density and C:N ratios. The moisture content was slightly higher than optimal - ranging from 66.3 to 66.6 percent - indicat- ing that a lower bulking agent to food ratio is not advisable, unless organic residuals have a moisture content significantly lower than 80 percent. Adding a dewatering step would achieve a lower moisture content. Al- ternately, drier bulking agents might be em- ployed t o reduce the ratio of nonfood materi- als in the mix.

Each pile achieved temperatures greater than 55'C at all monitoring points within 72 hours of formation. The rapid rise indicates that the mix characteristics were suitable. Initially, the piles had a low pH ranging from 4.0 to 4.6. which is considered to be subopti- mal. That was a reflection of even lower pH of the food residuals (3.5) and the yard trim- mings (3.6). The acidity was attributed to storage, albeit short term, under oxygen lim- ited or anaerobic conditions. After two weeks of composting, the pH of all piles rose to the neutral range (6.1 to 7.5) - a condition that was maintained thereafter.

During the first five weeks of composting, considerable moisture was lost through evaporation, the result of high heat genera- tion and aeration rates. Water was added to the piles several times over that period. Af- ter that point, water was not needed. A total of about 50 gallons of water/wet ton of the initial mix was added to the ASPS. The ATWsl as a result of frequent turning, lack of insulative cover, and a higher surface area to volume ratio, required approximately twice as much water (100 gallondwet ton of initial mix).

As anticipated, the piles with pure yard trimmings as a bulking agent generated the most heat over the entire composting period, followed by the yard trimmings/paper mix, and then the wood mix (the high heat gen- eration of the yard trimmings is attributed to its high energy content). Overall, the tem- peratures of all six piles varied considerably throughout the process, due to the relative- ly small mass of the piles and adjustments in the aeration rate. Small piles with their big surface to volume ratios lose heat more quickly than large piles. Pile turning and small changes in frequency and duration of l)!o:i-er. operation had a dramatic effect on pile temperature during the project.

The peak aeration demand (the basis for selecting the blower size) of each of the three mixes was similar. This observation indi-

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cates that in full-scale implementation, the selection of one of these three bulking materials would not affect the sizing of the blowers. The peak aeration rates of the ASP piles, how- ever, are about 1.5 times that of the ATW piles. For the spe- cific materials and operating conditions of this project, an aeration rate of 1,000 cu.ft./hour/dry ton of initial mix was found to be suitable.

The different bulking materials did not have an apparent effect on the processing period. The ATWs had slightly lower respiration rates than the ASPS, but the difference was not large enough to indicate that the ATW process requires a sig- nificantly shorter processing period than the ASP method. Based on the project results, a processing time of 10 to 14 weeks is adequate to produce a stable and mature product, contingent on maintenance of optimum conditions through- out the process.

Although the type of bulking agent did not impact the length of processing, the duration could be reduced if the amendment were removed from the composting pile at the point when the food residuals have stabilized. The use of recoverable bulking material (e.g. wood chips) is typically utilized with an ASP pro- cess. Of the bulkmg materials used in the study, chpped pal- lets are removed most readily through screening.

Overall, each of the bulking materials performed well and could be used in full-scale implementation. However, there are some differences worth noting. Although no special odor prob- lems were detected during the pilot, yard trimmings have more odor generating potential than mixed paper or wood. This is a special concern during the spring and summer when yard materials include a high concentration of grass clippings.

The project indicates that incorporation of paper at ap- proximately 40 percent of the initial mix volume (25 percent on a dry weight basis) is technically feasible. The paper read- ily decomposed during the 16 week process and did not neg- atively affect the composting process or final product quality. The paper is particularly effective in absorbing the free liq- uids associated with food trimmings, especially if collected commingled with the organic materials. That provides a means of managing the liquids at the source, improving stor- age and collection conditions.

The wood waste mix requires more attention in design and operation than the other bulking materials. Wood generates less heat during the composting process, and as a result, the ATW wood waste pile did not maintain pathogen reduction temperatures for the required 15 day period (EPA biosolids composting standards were used). The maintenance of pathogen reduction temperatures for the woody ASP pile was achieved, however (for ASP, a shorter duration of three days is required). Another concern with the wood bulking materi- al is that the freshly ground pallet portion resisted absorp- tion of moisture from the food residuals.

A potential implementation scenario entails the use of a screenable chipped wood bulking agent, such as pallets, in an ASP process. After three to four weeks of active composting, approximately 70 percent of the material would be removed through screening. The study suggests that this process would readily maintain pathogen reduction temperatures for the required period. While water absorption may be inade- quate initially, requiring an adjunct such as sawdust or shredded paper, experience indicates that wood chips become more absorptive after the first use.

ENVIRONMENTAL CONTROLS Process controls, especially the maintenance of aerobic con-

ditions, were a primary means of controlling odors. The fre- quent determination of oxygen levels within the compost piles found aerobic conditions were being maintained. There- fore, the aeration system and porosity of the initial mixes were adequate for distributing air through the piles.

Most odorous air generated was removed by the aeration system and treated with a biofilter. The results of quantita-

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tive and qualitative monitoring indicated that odor genera- tion was relatively low. A performance evaluation found that the biofilter removed 95 percent of detectable odors. Due to the negative aeration system, odor levels in the buildings were minimal. The biofilter proved to be an effective, low maintenance means of controlling odors, indicating that it would be an appropriate technology for full-scale food resid- uals composting.

Several aspects of the facility design and operations helped control nuisance animals and pests. The building enclosure significantly limited rodent and bird access. In addition, building doors were kept closed as much as possible.

The ASP'S insulative cover provided additional protection from rodents and insects. In the ATW piles, frequent turning dur- ing the initial two to four week period limited the potential for insect and rodent proliferation. Furthermore, turning facilitates the mixing of the cool outside lay- er into the hot interior, resulting in death of insect larvae and pu- pae. Additional vector controls included bait stations for trap- ping rodents and fly /

. .., . .. .~,..

paper strips.

tivity was minimal. This can be attribut-

Nuisance pest ac- - .

con t ro l s a t a ful l - scale facility should Food residuals are mixed with be targeted at deliv- bulking materials at the site, and

ery9 pile mixing and cornposting buildings with a front- the first week of com- end loader. posting.

Leachate production from the piles was limited by using bulking materials to absorb free liquids. The moisture con- tent of the initial mixes, although higher than optimal, was sufficiently low to limit generation of leachate. The volume generated in both buildings was approximately 10 gallons. However, a full-scale facility should provide a means of cap- turing liquids from incoming food residuals.

PRODUCTION AND QUALITY The ASP process yielded approximately 3.5 cu.yds. of

screened final product per dry ton of food residuals. The ATW piles yielded a smaller quantity - 1.3 to 3.1 cu.yds. The high- er yield from ASP is an apparent result of the insulative cov- er being incorporated into the final product.

The final compost products were of high quality. Each was well within local, state and federal criteria for trace metal content. The levels were less than half of the U.S. Environ- mental Protection Agency regulatory standards for unre- stricted use of biosolids compost, for example. Except for the ATW pile with the wood waste, the compost products met hu- man pathogen reduction standards. The products were of

placed in windrows inside temporary

equal or better quality than local competing products, and likely would be received fa- vorably in the marketplace. In particular, nitrogen content of the food residuals com- post was 1.5 to two times that of locally pro- duced yard trimmings compost.

I n thc ATW piles, the nitrogen, phospho- rous and potassium content of the product using only yard debris as a bulking agent is greater than the compost using yard trim- mingslmixed paper, which is higher than ., .> . , . *.." .;"' the wood waste based . .. ~' . . ?,;: mate r i a l . Tha t ou t -

come is to be expected, given that yard trim- to mings are higher in nutrients than mixed paper or wood chipsf sawdust. However, if recoverable wood chips are used, it could re- sult in a higher nutri- ent content, given that about 70 percent of the bulking agent is recov- ered in the process . The variation in nutri- ent levels was not ob- s e rved i n t he ASP piles, and may be a re-

sult of the insulative cover. The composting process does affect the

physical characteristics of the final product. The ATW composts are less dense, have a higher moisture holding capacity and small particle size. Windrow turning appears to improve the physical characteristics of the product. However, the coarse, dense screen overs used as the insulative cover in the ASP process contributed t o the less desirable physical characteristics of the product using that method. In a full-scale facility, final product would probably be used as the insu- lative cover, improving the physical charac- teristics of the compost.

The bulking material appears to have some impact on the chemical and physical characteristics of the products. Compost made with yard trimmings had consistently 1: izhr moisture holding capacity, smaller particle size, lower density and higher cation eschaxge capacity than the products made G r h t h e other two bulking agents. rn Gerry Croteau is a consultant with E&A Envi- ronmental Consultants Inc. of Bothell, Wash- ington, which was involved with implementa- tion of the collection and composting aspects of the project. Others assisting in this phase of the project include Christine Luboff of the Seattle Solid Waste Utility, Cedar Grove Composting, :7mc>mld City Recycling, Emcon Northwest, R. Cave Associates, Sound Resource Management Group Inc. and Alpha Engineers. More de- tailed compost testing results are available in "Volume I\,-: Compost Quality Testing," pre- pared by HcJrrera Environmental Consultants and Sound RL.source Management Group in co- opcratiort 1c.ith E&A Environmental Consul- tants. Thc study urns funded by Washington "':/!(,. ("it?. o/'Smttlc and King County.

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